EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 249 



on induction, and the whole responsibility for the appearance of the main 

 embryonic axis can be attributed to this process and to the self-individua- 

 tion (i.e. regionaUsation) of the mesoderm. 



It seems therefore that all the groups of vertebrates employ essentially 

 similar epigenetic mechanisms. The main changes which have occurred 

 within the group appear to be: (i) The swamping of the initial structure 

 of the oocyte by the enormous quantities of yolk laid down in fish and 

 birds, and a consequent postponement of the time at which the plane of 

 bilateral symmetry is fmally determined, and (2) the interpolation of a new 

 causal relationship between endoderm and mesoderm in birds (and 

 possibly in reptiles and mammals) in. consequence of the difference in the 

 time of invagination of the two layers. 



SUGGESTED READING 



Needham 1942, pp. 320-30; Dalcq 1938, pp. 7-58. For comparative gastrulation, 

 Pasteels 1940, Waddington 1952^ or I952fl, pp. 39-50. For teleosts, Oppenheimer 1947; 

 for mammals Gluecksohn-Schoenheimer 1949, Dalcq 1951a. 



