EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 225 



mesoderm does not extend right round the egg but on the ventral side 

 the presumptive ectoderm comes into direct contact with the presumptive 

 endoderm. This situation, which would constitute a considerable differ- 

 ence from that of the Ampliibia, has been questioned by Pasteels (1940) 

 who points out that in the early neural plate stage, when invagination is 

 still proceeding around the blastopore, this structure is undoubtedly sur- 

 rounded by a complete ring of mesoderm, some of which is being in- 

 vaginated over the ventral lip at this time. Although Pasteels' material was 

 not sufficient to enable liim to draw an alternative map of the presumptive 

 areas in the early gastrula, he concludes that the mesoderm must in fact 

 extend right round the egg between the ectoderm and endoderm. He 

 suggests that the main difference between the gastrulation of the lamprey 

 and the frog is that, in the former, the invagination of mesoderm over the 

 lateral and ventral lips is delayed so that it does not begin until the main 

 mass of endodermal material has already passed into the interior. The two 

 processes in fact differ only in this comparatively minor matter of timing. 



Some attention has also been paid to the causal embryology or epi- 

 genetics of these forms. It was shown as long ago as 1900 by Bataillon 

 that if the first two blastomeres are separated each may give a complete 

 embryo. The experiment has been repeated more recently (Montalenti 

 and Maccagno 1935; Bytinski-Salz 1937^) and it has been shown that the 

 situation is almost exactly the same as that revealed by constriction experi- 

 ments in the Amphibia. Further, fragments from the dorsal lip of the 

 blastopore can be grafted just as in nev^s (Bytinski-Salz 1937&; Yamada 

 193 8). When they come in contact with gastrula ectoderm they cause the 

 induction of a secondary neural axis and, so far as experiment has gone, 

 repeat in every way the behaviour we have seen in Urodele material. 



Unfortunately there has been little work by modern methods on 

 other groups of primitive fish. Those types wliich have markedly telo- 

 lecithal eggs and very unequal, but still total, cleavage would be particu- 

 larly interesting as providing a transition to the higher forms, which are 

 provided with very large masses of yolk and develop through the stage 

 of a blastoderm. No vital stain experiments seem however to have been 

 made on these transitional types. Ginsburg and Dettlaff (1944) bave 

 shortly reported a small number of experiments on the embryos of the 

 sturgeon Acipenser. This has an egg, about 3 mm. across, which is rather 

 amphibian-like in the cleavage and early gastrulation stages. Grafts of the 

 blastopore hp showed that it behaved as an organiser and could induce 

 secondary neural folds in a host embryo. The donors from which the 

 blastopore lip had been removed developed no nervous system or axial 

 organs, and the authors suggest that the presumptive materials for these 



