Taking the mammalia as a whole, there are two principal forms of digestion; the first is by means of the juices which the wall of the gut secretes, and which reduce the food to a form absorbable by the wall of the alimentary canal; the second is by means of organisms contained in the alimentary canal, which not only can perform a similar function, but which, by multiplying in the gut at the expense of the foodstuffs, can themselves form diet more nutritive than that in which they grow. In one way or another the organisms can convert materials in their food, which otherwise would be wasted, into articles of diet important to the animal in whose gut these lowly creatures thrive.

The problem to be presented here emerges from the following groups of facts and more or less generally accepted opinions.

As heterochromatin we may define those parts of chromosomes which reach maximum nucleic acid charge in mitosis or meiosis in times other than metaphase. In salivary gland chromosomes (which are more conveniently called polytene chromosomes) of Drosophila melanogaster the proximal heterochromatic parts of all chromosomes come together and form a central undifferentiated mass, the chromocentre. Genetically heterochromatin forms the so-called inert regions of the chromosomes.

The mechanism by which irradiation produces gross re-arrangements of chromosomes (inversions, translocations, deletions) is now generally believed to occur in two steps: (a) breakage of one or more chromosomes, yielding two or more points of breakage; and (b) reunion of broken ends in a novel combination. Thus the frequency with which a given treatment produces re-arrangements depends not only on the number of breaks produced, but on the circumstances under which reunion takes place. In Drosophila the fertilized ovum seems to offer particularly favourable conditions for breaks produced in the mature spermatozoon to join up into new combinations. In immature germ-cells, on the other hand, re-arrangements are much less readily produced by irradiation (Muller, 1930; Shapiro and Neuhaus, 1933; Shapiro, 1937; Glass, 1939). Evidence for the occurrence of spontaneous or induced re-arrangements in somatic cells of Drosophila is very scanty.

A Summary of the facts with numerical details omitted can be given as follows. In the diatoms—

1. Species-lifetimes have been alternatively short or long.

2. The main ratios of shorts to longs, or evanescents to permanents, e/p, are 3:1 for Centrales, 10:6 for Fragilariaceæ, 7:9 for allogamous Pennales. These ratios or nearest approaches to these ratios occur in the above three groups, (a) in each group as a whole; (b) in species-arising in each group per geological subperiod; and (c) in heterogeneous genera in each genus as a whole and in each genus per subperiod. A derivative 1:3 ratio also occurs in some pennate genera. There are some significant and understandable exceptions.

During a study of the micro-structure of keratin fibres (Stoves, 1944) it was found that certain members of the Mustelidæ, e.g. kolinsky (M. Sibirica), ermine (M. erminœ), and Jap mink (M. itatsi), possess primary hairs with well-developed cuticle, cortex, and medulla. In the case of rabbits and hares (Leporidæ), however, the corresponding fibres are composed almost entirely of medulla cells. The two types of fibres, therefore, provide suitable material for studying the chemical reactivity of the keratin constituting cuticle, cortex, and medulla of animal hair. Accordingly, the effect of various reagents upon transverse sections of fibres from the above-mentioned animals was examined, and a preliminary report of the results has already been made (Stoves, 1943 a). A more detailed account of the work now appears.

The work recorded in this paper was undertaken in order to obtain further knowledge regarding the behaviour of the cytoplasmic components of the male germ-cells of mammals and the contribution made by these bodies to the structure of the ripe sperm. Consequently, mammals from different groups were examined.

Investigations were carried out independently by Zlotnik on the male germ-cells of the dog, the cat, and the rabbit, by Gresson on the boar and the ram, and jointly by Gresson and Zlotnik on the white rat and the golden hamster. Zlotnik (1943) originally identified and described the nuclear-ring of the spermatid and sperm of the dog. The accessory body was recognised simultaneously by him and by Gresson, but Zlotnik was the first to observe an accessory body within the localised Golgi material of the spermatocyte and to follow its extrusion to the cytoplasm, and, later, Gresson identified an accessory body within the Golgi material of the spermatid of the boar. In general the conclusions of the two authors are closely similar, such differences of detail or of interpretation as exist are discussed in this paper.

Studies of the inheritance of resistance to two strains, A and B, of the blight fungus (Phytophthora infestans) in derivatives of the triple hybrid (S. Rybinii x S. demissum) x S. tuberosum and in hybrids descended from S. demissum x S. tuberosum led to the conclusion that resistance was controlled by two genes, designated Ra and Rb, of different phylogenetic origin, the former conferring resistance against strain A and the latter against both strain A and strain B (Black, 1943).

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.

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.

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.

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.

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.