454 



Special Vertebrate Organogenesis 



derm, occurring at different times for dif- 

 ferent organs (Rudnick, '35). By means of 

 vital dyes, a cellular migration from ecto- 

 derm to entoderm has been demonstrated in 

 correlation with the shift of gut potency 

 from ectoderm to entoderm in the early 

 blastoderm (Hunt, '37b), but there is "no 

 evidence that the localization process itself is 

 to be explained on the basis of a cellular 

 migration" (Rvidnick and Rawles, '37). 



Mouth Formation. Experiments on the 

 stomodeal region of A. punctatum by Adams 

 ('24, '31) showed that mouth invagination 

 is dependent upon contact of the ectoderm 

 with the underlying entoderm. Experiments 

 by Balinsky ('47b) confirmed and extended 

 the view that the entoderm acts as an in- 

 ductor on the stomodeal ectoderm. Most of 

 Balinsky's experiments were done on T. 

 taeniatus embryos in late gastrula and early 



Fig. 166. A and B, sketch of operation for removal of whole entoderm at neurula stage. C, entoderm-free 

 larva of Triturus alpestris 20 days after operation. (Redrawn after Nieuwkoop, '46.) 



The prospective potencies of the different 

 digestive tract primordia are greater than 

 their individual prospective fates. This is 

 shown by an overlapping of ectodermal and 

 entodermal gut-potency stages (cited above) 

 and by the extent of gut-potency tissue on the 

 early blastoderm (Butler, '35; Rudnick and 

 Rawles, '37). 



Complete extirpation of the entoderm 

 from neurula stage amphibian embryos in- 

 dicates that the entoderm has striking effects 

 on the development of other tissues (Nieuw- 

 koop, '46). Entoderm-free embryos frequently 

 develop two pairs of forelimbs (Fig. 166), 

 they show various degrees of microcephaly 

 and cyclopia, they lack gills and, as described 

 in a previous section, there is a failure of 

 heart development. 



neurula stages. He found that ectodermal 

 mouth invagination usually failed after com- 

 plete removal of mouth entoderm, and that 

 presumptive mouth ectoderm transplanted 

 to abnormal locations prodviced a mouth in- 

 vagination only when underlying mouth 

 entoderm was transplanted along with the 

 presumptive ectoderm. A regional factor in 

 the stomodeal ectoderm was indicated by 

 the following experiments: (1) mouth ento- 

 derm transplanted to foreign positions did 

 not induce mouth invagination in the over- 

 lying foreign ectoderm; (2) mouth invagina- 

 tion failed when mouth ectoderm was com- 

 pletely replaced by foreign ectoderm. In the 

 latter result, the experiments on Triton ap- 

 pear to differ from those on Amblystoma by 

 Adams ('24), where mouth parts were found 



