EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 24I 



Dunn (1941) and in particular Gluecksolin-Schoenheimer (1949, 1953) 

 working on the mouse, have made very important contributions in this 

 way. 



The first genes from which important embryological consequences 

 could be deduced had their most obvious effects on the tail. The Brachyury 

 gene T is a dominant, the heterozygous mice having short tails. The Tj T 

 homozygotes die before birth and in the embryos the whole posterior 

 region of the body is missing (Chesley 1935). Detailed investigation 

 showed that no notochord (or almost none) is ever formed in these homo- 

 zygous embryos, and Chesley suggested that this was the primary action, 

 the effect on other structures such as the nerve-cord being the result of 

 secondary reactions of an inductive nature. This was confirmed, and the 

 evidence made more convincing, when Gluecksohn-Schoenheimer in- 

 vestigated embryos of animals wliich were heterozygous both for T and 

 for one or other of the genes t^ and f^. These embryos have normal- 

 looking tails till an age of about eleven days, after which the tail becomes 

 constricted at its base and degenerates, the young being quite tailless when 

 bom. Histological examination showed that even in the apparently nor- 

 mal tails of early stages, no notochord is present, although the neural tube, 

 somites and tailgut are formed as usual. It seems then that the later degen- 

 eration is a consequence of the lack of the notochord, and some 'inductive' 

 relation is indicated. It should be noted, however, that the relation is not 

 quite that of evocation ; even in the absence of the notochord, the neural 

 tube is formed, presumably induced by the remainder of the mesoderm, 

 but it is unable to persist. There is no exact parallel to this in other verte- 

 brates, since the operative removal of the notochord, e.g. in Amphibia, 

 does not lead to the regression of the neural tube, but only to a failure of 

 its normal elongation and an alteration of its usual cross-sectional shape. 



The homozygous t^/t^ embryos show more profound abnormalities. 

 The whole endoderm tends to hft away from the rest of the inner cell 

 mass some time before the beginnings of embryo formation are visible. 

 The cells of the mass remain alive for some time, and some growth takes 

 place, but there is no organisation of a primitive streak and no mesoderm 

 appears; a day or two after the onset of the condition, the embryos die 

 and are resorbed. Gluecksohn-Schoenheimer suggests that this may indi- 

 cate that in the mammals, as in the birds, the endoderm may play an 

 essential role in inducing the formation of mesoderm. 



The same author has described a still more interesting gene, known as 

 Kink. The heterozygotes show various spinal and tail abnormalities, and 

 the homozygotes die at about nine days after fertilisation. Before death, 

 a considerable amount of development has occurred, and the embryos 



