The frequent reports of 20 per cent, or more losses due to neoplastic diseases in poultry flocks indicate that this is one of the greatest troubles of the industry. Though certain of the avian tumours have been favourite subjects for experimental cancer research, little attention has been directed to the problem of the ætiology and control of avian cancer itself. Consequently, despite the urgency of the problem, there is little information available which can be used as a basis for prescribing any preventive measures. It is not even known whether the majority of the spontaneous tumours are induced by a virus. Not many attempts have been made to transplant and study these tumours, considering the wealth of material available; and only a small proportion of the attempts have succeeded. Though most of these successfully transplanted sarcomas were found to be associated with a causative virus, it is impossible to state whether this is the usual condition of spontaneous tumours, or whether the few successful transplants were aberrant forms whose associated virus altered the growth characteristics and thus enabled a successful transplant to be made more easily.

The Acrididæ are characterised by a very uniform karyotype. With a few exceptions, the species so far studied contain 23 chromosomes in the male, which are very similar in size and structure. Attempts to deduce the probable mechanism of speciation within the Acrididæ were restricted only to metrical studies of chromosomes. Recently, however, Coleman (1943) has established in two species the subterminal position of the primary constriction or centromere and the variation in the length of the two arms in different chromosomes of the same karyotype. In the present paper additional evidence is presented which shows a linear differentiation of some chromosomes of the Acrididæ. Furthermore, in view of the fact that chromosome behaviour during meiosis is a more reliable criterion of ascertaining linear differentiation, the meiosis was studied in three genera.

Three species belonging to three different genera were studied. They are Spathosternum prasiniferum Walker, Oxya sp. and Phlœoba sp. The former two belong to the sub-family Catantopinæ and the latter one to Acrididæ. Adult male specimens have been captured from the field adjoining the Biological Departments, Calcutta University, mostly in the months of August and September 1943–44. The testes were dissected out in Ringer's solution (cold-blooded formula) and fixed in Medium Flemming, Belling's, and Kahle's fixatives. The two latter were very suitable for smear preparations. Sections were cut at 25 to 30 micra in thickness and stained in iodine-crystal violet and in Heidenhain's hæmatoxylin. A 2 per cent, solution of safranin also gave good results. Squash preparations were stained by Feulgen's method. Diplotene stages in Phlœoba were studied from temporary aceto-carmine preparations.

According to Grollman (1936), there is no evidence for an intimate relation between the gonads and the adrenal proper, i.e. the cortex apart from the “fœtal zone” (Grollman, 1942). Recent papers, however (Bourne and Zuckerman, 1940 a, 1940 b; Schilling and Laqueur, 1941; Selye and Albert, 1942; Selye, Rowley, and Hall, 1943; Sarason, 1943, and others), confirm the opposite view (Kolmer, 1912; Korenchevsky, Dennison, and Simpson, 1935). The results, especially those concerning the action of gonadal substances on the adrenal cortex, vary as between different authors and according to the species of experimental animal used. Grollman (1936), for example, states that the cortex is not influenced by œstrogen. Allen and Bern (1942), however, and Golla and Reiss (1942) report a change in the cortex associated with an increase of adrenal weight. The former authors found an increase in lipoid-containing vacuoles in guinea-pigs treated with stilbœstrol; the latter, on the other hand, observed a lipoid decrease in rats treated with œstrone. Comparison of these results with others which they obtained with hypophysectomized rats even led Golla and Reiss to postulate the existence of two corticotrophic principles in the hypophysis: one controlling the distribution of lipoid in the cortex and the other the adrenal weight.

The main interest in the healing of an injured nerve centres, naturally, on the regeneration of the axon. As the birthplace of the new fibres the central stump has attracted and received considerable attention, and the histology of the peripheral stump has been very fully investigated, particularly by Cajal (1928), and more recently by Young (1942), Weddell and Glees (1941), and Holmes and Young (1942). In contrast to our accurate knowledge of conditions in the central and peripheral stumps, there are scant records of the progress of repair at the injured region when the lesion results in the total destruction of the nerve fibres.

In the course of experiments on rabbit sciatic nerves designed to test the effect of interference with the blood supply on the regeneration of injured nerve (Bacsich and Wyburn, 1945 a and 1945 b) total degeneration was accomplished by a severe crush with artery forceps. This gave us an opportunity to study the early histological changes in the crushed areas of these nerves.

Since it is recognised that a female rodent will litter best if left alone, it is rare for breeders to see the placenta, which is usually devoured by the mother immediately after the birth of the young. Work upon the milk factor for mammary carcinoma in mice has necessitated frequent examination of mice about to litter in order to exchange litters as soon after birth as possible, and thus a number of placentas have been seen when the mouse was caught in the act of littering. Nevertheless these represent only a small proportion compared with the number of mice born during this work. It is of some interest that two cases of placental fusion have been noted among this small series, which suggests that this condition may be fairly common among mice. A rough estimate of the frequency in this series is about 1 per cent. No reports of similar cases have been found in the literature, though a note by Merton (1938) that two embryos had been “implanted in the same place” may refer to a similar case. Because of the implications of this finding in relation to sexual and genetical differentiation in the mouse, a description of this material seems justified.

The Bunsen-Roscoe law states that the effect of radiation is always the same regardless of the variation in the intensity or in the time of radiation as long as the product of the two is kept constant. This law holds true for many ordinary physico-chemical reactions over a wide range of intensities. The present investigation was undertaken to test the validity of the Bunsen-Roscoe law by studying the frequency of (1) sex-linked lethal mutations and (2) translocations in Drosophila melanogaster at much lower intensities of radiation than those previously tried.

Heterokaryosis is the condition in which one cell of a mycelium contains two or more genetically different nuclei. Hansen and Smith (1932), working with Botrytis cinerea, which has multi-nucleate spores, showed that single-spore cultures of this fungus sometimes contained two types of nuclei and that such heterokaryotic types could readily be induced by hyphal fusion between different homokaryotic types. These authors analysed individuals from thirty genera of imperfect fungi and found more than half of them to be heterokaryotic. It seems, therefore, that heterokaryosis is a widespread phenomenon among such fungi.

N. crassa is a heterothallic fungus with an eight-spored ascus and a pair of well-defined mating-type factors which have been very generally called sex factors. Perithecia are formed only when cultures of the opposite mating types are brought into contact. Both mating types can produce ascogonia, generally conceived to be female organs, and microconidia, sometimes called spermatia (Dodge, 1935), which may correspond to male gametes. Therefore it is doubtful whether the mating-type factors are true sex factors.

A non-committal term such as “mating type” or “incompatibility factor” is preferable. However, because of the convenience of the terms associated with sex, such as “unisexual” and “bisexual”, I shall follow previous authors (Dodge, Lindegren, Beadle) in using these terms with reference to the mating-type factors, at the same time emphasising that these factors may not be sex factors in the generally accepted sense of the term.

During the winter of 1937–38 it became necessary to obtain the maximum number of offspring from a certain pen mating for the purposes of an experiment unconnected with the present report. Accordingly all suitable eggs were incubated, and since chicks were required all the year round the data which accumulated related to periods extending beyond the normal hatching season. Not long after the beginning of the project strikingly consistent differences between the records of the individual females became apparent, and interest in this phenomenon led to the decision to incubate every intact egg from the pen, irrespective of any normally undesirable characteristics of shell texture or shape. This was done for two years (1938–39), and provided the main data for the study of fertility and embryonic mortality reported here.

It was in 1878 that Ehrlich, with the aid of his triacid stain, described in terms of granulation of the cytoplasm six types of leucocytes. This marked the first real advance in our knowledge of the blood corpuscles. Present-day classification of cells in the blood-forming tissues gives six main groups with about thirty named types of cells. These fine subdivisions appear to represent different stages in different lines of development from one original type, chiefly by change in the shape of the resting nucleus. Divergence in opinion arises as to whether all these types of cell develop in the red bone marrow, or whether some arise elsewhere in the body.

An account was previously given of the discovery of two cases of placental fusion in mice, and it was suggested that this condition might occur fairly frequently. This has been amply borne out by subsequent experience, as fourteen other examples have been found while working with mice, and in addition it has once been found in the rat. From this additional material further information about the types of developmental abnormalities that may result from this condition has been gathered. Since the last paper, Owen (1945) has found that commingling of the fœtal circulations in cattle twins can result in the transfer of erythrocyte precursors between partners, resulting in animals exhibiting blood-groups bearing no relation to their genotype. Similar transfers in mice would lead to confusion in many genetic experiments, especially in transplantation experiments with tumours.

No genetical basis for the condition is apparent from the data available. It has been, noted in three matings of CBA mice, two of C3H mice, eight C3♂ × CBA♀, and three stock mice, the proportions being roughly related to the relative numbers of each type examined at the time of littering. The frequency is rather less than 1 per cent, of all placentas seen. The only common feature, to which attention was also drawn in the previous paper, has been that the fusion occurred always in conditions of uterine crowding due to large litter size. The smallest litter size in which this fusion was found was five, but in this case five placentas were present in one horn. Whenever labour was observed, it appeared to be difficult and prolonged. Once, by good fortune, delivery was timed approximately. A CBA mouse was found to have delivered two young, and two more were born within the next ten minutes. Nothing further appeared for two hours and ten minutes, when a live pair with fused placentas was delivered with some distress. The last two of the litter, making eight in all, were delivered within twenty minutes of the completion of the birth of the fused pair

The discovery (Auerbach, 1943, 1946; Auerbach and Robson, 1946, 1947) that mustard gas is comparable to X-rays and similar physical agencies in its ability to produce mutations and chromosome rearrangements has opened up a new line of approach to the problem of gene mutation. It is to be expected that a comparative study of the mechanism by which chemical substances on the one hand and physical agencies on the other exercise their mutagenic effects, will further our understanding of the process of mutation itself. One of the first questions to be tackled in the early days of radiation genetics was the possibility of a delayed mutagenic action of irradiation (Muller, 1927; Timoféeff-Ressovsky, 1930, 1931; Grüneberg, 1931). The bulk of the evidence (see, however, Bishop, 1942) indicates that X-ray-induced mutations and chromosome breaks arise as an immediate effect of the irradiation, although after treatment of mature spermatozoa new recombinations of broken chromosomes may be delayed until the spermatozoon has entered the egg. Data obtained by Stadler (1939) suggest that after ultra-violet radiation of pollen grains the mutational process often is not completed before the treated chromosome has split into its two daughter chromatids. This results in a high proportion of mosaics. A similarly high proportion of mosaics has been found in the progeny of Drosophila ♂♂ which had been treated with mustard gas (Auerbach, 1946; Auerbach and Robson, 1946). This raises the question of a possible delayed action of the chemical mutagenic treatment.

Intravenous injection of posterior lobe pituitary extracts into spinal or anæsthetised mammals has several effects. One is to elevate blood pressure. Two animal preparations are commonly used for the biological assay of this activity. Most American workers seem to favour a dog deeply anæsthetised with chloretone or chloralose. 10 per cent, accuracy is claimed. We have no experience of this preparation. There have been very few tracings published and those that we have seen do not inspire confidence. This apart, a method employing dogs is unlikely to find favour in this country because of the difficulty of obtaining, feeding, and housing suitable animals. Most workers in this country use the spinal cat. As originally described by Dale and Laidlaw (1912), the method employing this preparation was insensitive and open to grave errors. Hogben et al. (1924), after intensive study, showed that it could be sensitive, reliable, and susceptible of good discrimination. The Hogben procedure has been generally adopted.

Some years ago the British Association for the Advancement of Science standardised the methods for the determination of the measurements of length, breadth, and height of the living head or dead skull, but said nothing about the brain. But to-day we know that these same methods can also be used for the measurement of the post-mortem brain. The advantages of having one common series of measurements for the living head, dried skull, and post-mortem brain, and one standard mode of comparing the results are so obvious as to need no further justification.

The classical view that, of the two known types of nucleic acid, desoxyribonucleic acid (thymonucleic acid) is characteristic of animal tissues and ribonucleic acid (pentose nucleic acid) of plant tissues, was established by the work of Kossel and others and upheld as a fundamental generalisation by Jones (1920). This view, which was current for over thirty years, has had to be abandoned in the light of more recent evidence. Not only has desoxyribonucleic acid been found in plant tissues, but ribonucleic acid has been isolated from the pancreas (Jorpes, 1928, 1934) and has been shown to be widespread in animal tissues (Davidson and Waymouth, 1943, 1944 a, b). Ribonucleic acid has been isolated, also, from liver tissue, in which it is present in much larger amounts than is desoxyribonucleic acid (Davidson and Waymouth, 1944 c).

According to Borst (1913) each individual should be regarded as a specific biochemical system, and within this common background the different organs and tissues work together whilst preserving their own characteristics. On such a foundation, strengthened by the results of his own extensive experiments, Loeb (1930,1945) has built up his conception of the biological basis of individuality. According to his thesis the individuality of a tissue is a summation and integration of qualities in respect of its identity as a particular tissue or organ, in respect of the organism of which it is a part, and in respect of the species and order of this organism. The autograft is attuned to the biochemical system of the organism and is therefore accepted as a transplant, but the host reacts to the homograft (and more so to the heterograft) and makes an effort to destroy it. While the extent of the host reaction depends on many factors—e.g. genetical relationship, age of host and donor, etc.—the ability of different organs and tissues to survive as homografts varies considerably. It is, for example, well known that homografts of cartilage and cornea survive within the host long after homografts of other tissues and organs have been destroyed.

To regard the more prolonged survival of homografts of cartilage and cornea as evidence of low tissue specificity (Loeb, 1930, 1945) still leaves undetermined the real reason why these particular tissues are less readily overwhelmed by those of the host.

Muller's classical discovery in 1927 of the mutagenic action of X-rays has provided an extremely useful tool for studying the nature of gene mutation and chromosome breakage. It has always been realized that if, in addition, chemical substances were found capable of producing mutations and/or chromosome breaks a further important step forward in the analysis of the mutation process would follow. The search for such mutagenic substances has therefore been going on in various laboratories for some ten years, until recently, however, without definitely positive results. Only during the last few years has it been shown that certain chemical substances, such as mustard gas (ββ′-dichlordiethylsulphide, (ClCH2 · CH2)2S), may be as effective as X-rays in producing mutations and chromosome breaks (Auerbach, 1943; Auerbach and Robson, 1944; Auerbach, 1945; Auerbach and Robson, 1946). Indeed, the similarities between the results of these two types of treatment are impressive. In the course of more than four years during which work of this kind has been carried out by the author, only few differences between the genetical results of chemical treatment and of irradiation have come to light. Particular interest attaches to these dissimilarities, because they, rather than the many similarities, may throw some new light on the process of mutation and through this on the nature of the gene. One of the few striking dissimilarities is the high frequency of certain types of mosaics which can be produced by chemical treatment. It is the object of the present paper to summarize the data on chemically induced mosaicism, contrast them with similar data from X-ray experiments, and discuss the similarities and differences from the point of view of the mechanism of induced mutation.

Examination of mammalian pelts reveals the presence of two types of hair—long, glossy fibres known as “guard”- or “top-hair,” and shorter, softer, more curly fibres termed ”ground”- or “under-wool.” Guard-hairs are of two kinds: primary fibres classified by Toldt (1910) as “Leithaare,” and shorter, less robust secondary fibres which Toldt described as “Grannenhaare.” The under-wool is analogous to Toldt's “Wohlhaare.” More recently Toldt (1935) has described the appearance of the various fibre types occurring in hair from a wide range of mammals, and gives an extensive bibliography on hair from many orders of mammalia, including the two at present under consideration. Glaister (1931) made a photomicrographic survey of whole mounts and transverse sections of fibres from a large number of mammals. This work, however, suffers from the disadvantage that the photomicrographs were taken at low magnifications. When photographed at magnifications of the order × 500, transverse sections of fibres readily demonstrate the distribution and structure of cuticle, cortex and medulla. Such information is valuable as a means of studying structural variations in hair from different types of mammals, and often possesses considerable taxonomic significance and practical value—Dearborn (1939) reports the use of cross-sections of mammalian guard-hairs as a help to identifying small hairs found in the stomachs and excreta of predators, while the importance of fibre histology in forensic science has been reviewed by Smith and Glaister (1939). Furthermore, microchemical tests carried out on wool fibres (Bekker and King, 1931) and on transverse sections of kolinsky and hare fibres (Stoves, 1943 a) have shown that cuticle, cortex, and medulla exhibit different chemical properties, so that morphological differentiation is accompanied by variation in chemical reactivity.

Congenital transposition of the viscera has frequently been recorded in man. Figures relating to its incidence are quoted by O'Farrell (1941) and by Kean (1942), and the possible causes of reversal of symmetry are discussed by Cockayne (1939). A case of situs inversus viscerum in the cat is described by Wragg (1939), and one case was observed in the mouse by Grüneberg (1943). So far as the writers are aware this condition has not previously been recorded in the rat.