246 



PRINCIPLES OF EMBRYOLOGY 



Pasteels. Its main merit is that it gives an example of one way in which it 

 is possible to envisage both the fish and bird blastoderms as two modahties 

 of one general process ; and further, it draws attention to the need to con- 

 sider the disposition of the presumptive areas in depth as well as on the 

 surface. It is most desirable that we should find out more about the loca- 

 tion of the presumptive areas in the extremely yolky types of amphibian 

 eggs, particularly those of the Gynmophiona. In these, much of the en do- 

 derm seems to be beneath the surface before gastrulation begins and to 

 become distinct by a process of delamination, which would seem to accord 

 better with Waddington's scheme than with the Dalcq-Pasteels' one. 

 Gastrulation in these eggs has, however, not been studied since Brauer's 

 work in 1897. 



ventral 



ventral 



dorsal 



dorsal 



O 



o o o o o 

 000 



Figure ii.ii 



A, the basic vertebrate map, with a line of folding ('the primitive edge') 

 indicated by a broken line. B, the blastula squashed dowoa on to a mass of 

 yolk, having folded along the primitive edge. If such a deformation is to 

 produce the bird map, there would also have to be an expansion of the epi- 

 dermal area. (From Waddington 1952.) 



6. Comparative causal embryology of vertebrates 



The various types of vertebrate eggs, different in many respects and yet 

 all anatomically related in the ways we have just discussed, provide a 

 wonderful opportunity for studies on the kind of alterations in epigenetic 

 mechanisms which evolution can bring about. There is no doubt that one 

 ought eventually to envisage evolution not as a process wliich merely 

 causes modifications in the structure of adult animals, but rather as one 

 by which the causal sequences of development have been changed. These 

 changes have affected even the basic mechanisms which operate in the 

 early stages of development. Dalcq [igsib) has referred to mutations which 

 affect these stages as 'onto-mutations'. Little is known in detail about them 

 from the genetical side, though many of the factors which are labelled 

 (and usually dismissed) as 'female-steriles' probably fall into tliis category 



