Young gastrulae of Triton alpestris were medianely transsected with a quick cut effected by means of a glass needle. On each of the open median planes of the two halves was grafted the whole presumptive epidermis of a second and, respectively, of a third gastrula in the normal animal-vegetative orientation (fig. 1). Each of the right and left twin combinations thus produced contained therefore all the organ rudiments in half the normal quantity - with the exception of the epidermis rudiment which appeared in one and a half times the normal quantity.
The transplanted epidermal rudiment heals in and afterwards takes part in the formation of the embryo. In the course of gastrulation and neurulation the halved blastopore lip completes itself out of the grafted material which participates in the formation of the nervous system and the mesoderm (including probably notochord and myotomes). On the whole, there arise in favourable cases, straight, well-stretched larvae. More frequently, however, are the larvae with incurvations, persisting yolk plug, or with Spina bifida (see Mangold and Testa 1953, figg. 8-a, b, and 9-a, b, and, above, figg. 2-a, 3-a, 10-a, 11-a). The causes of these deficiencies must be looked for in secondary damages occurring during the operation, in incomplete healing processes, and in disturbances of the gastrulation.
As a rule, a far-reaching regulation of the halved rudiments towards bilaterally built organs takes place, but even in favourable cases it could be observed that some organs were slightly underdeveloped on the inner sides of the twins (these are the completed sides). The head, beside such asymmetries, showed synophthalmous and cyclopean deficiencies in most of the cases.
14 Larvae (7 couples) were chosen out of a greater material to investigate on their microscopical sections the regulation of the inner organs. Marked differences were found to exist between the single organs in this regard (see table I, pag. 391). The best regulation was represented in the mesencephalon showing perfect symmetrical structure in most of the cases (figg. 2-b, 8-c, above). The greater part of the other organs could be practically symmetrical, too, in favorable cases with, however, varying frequency. The lowest degree of regulative power appeared in the myotomes and in the rhombencephalon. The most anteriorly situated myotomes were even exceptionless asymmetrical, i. e. completed but imperfectly (figg. 4-b, 7-c); for most of those situated in the trunk and the tail this is likewise the case, in spite of the late date of the formation of the latter (figg. 4-c, 4-d). Sometimes the myotomes of the two sides resulted to be unequal even when the outward appearance of the trunk and tail seemed perfectly symmetrical: it was then found that their structure was of a less compact nature on the completed sides (fig. 4-c). As to the rhombencephalon, it mosdy was distinctly asymmetrical (figg. 5-b,, 7-b), in contradistinction to the often well-developed symmetry of the spinal chord (figg. 2-c, 4-c).
Such differences of size between the two sides are known to occur also with human twins occasionally (Gedda 1951, Testa 1956).
In the six diagnosticahle cases it was found that all the right-hand members of the twins showed situs inversus viscerum, whereas with the left-hand partners only one inverted case was observed, this being a situs inversus cordis.
In the fold formations (poaches) of the transplanted epidermis could be induced the formation of supernumerary organs of an « according-to-region » type, as for instance noses, prosencephalon, eyes, lenses, balancers (fig. 11-b).
The causes supposed to be responsible for the cyclopean and lateral deficiencies are discussed (page 395).