EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 245 



in the blastodermal types, we fmd delaniiiiation, that is, a process by which 

 ail original single thick layer of tissue becomes rearranged into two thin- 

 ner separate layers. Until we know more of the mechanical causation of 

 the movements, it is not possible to make any further meaningful com- 

 parisons between them. 



The main aspect of the comparative scheme which one would like to 

 understand more fuUy is the relation between the two sorts of blasto- 

 dermal development. Two main ways of regarding this have been pro- 

 posed. Dalcq (1938) and Pasteels (1940), who were the first to discuss the 

 matter at all fully, were content to accept an irreconcilable duahty within 

 the evolutionary system of the vertebrates. According to their scheme, 

 the teleost map should be regarded as derived from that of the Amphibia 

 by the insertion of a large mass of yolk near the vegetative pole, while 

 the bird-reptile map is derived by inserting the mass of yolk somewhere 

 within the area of presumptive epidermis (at point B in Fig. 11 . 10). Tliis is 

 the simplest and most straightforward scheme, but the idea of such drastic 

 quahtative differences of egg structure within the vertebrates is not entirely 

 satisfactory, 



Waddington (1952&) has suggested that one ought to take into account 

 not only the disposition of the presumptive areas on the egg surface but 

 also their arrangement in depth. The blastodermal types might be derived 

 from the ampliibian arrangement, not by opening the latter at some point 

 and spreading it out so that it can sit as a cap on top of a large mass of 

 yolk which is inserted in the hole, but rather by imagining that the amplii- 

 bian blastula becomes yolk-free, and then that the whole sphere is squashed 

 down on to the surface of the separated yolk mass (Fig. 11. 11). If, when 

 the spherical blastula is distorted in this way, the line along which it 

 folds (the 'primitive edge') lies whoUy within the presumptive mesoderm, 

 then the blastoderm will have a mesodermal margin and the whole of the 

 endoderm will already be beneath the surface before gastrulation begins. 

 If the primitive edge on the dorsal side lies a little lower, within the eiido- 

 dernial region, some of the dorsal margin of the blastoderm will be endo- 

 dernial. According to the exact position of the line, one can easily arrive 

 at the conditions seen in the Selachia or teleosts. Again, if the primitive 

 edge lies liigher on the amphibian map, it will, particularly on the ventral 

 side, cut into the presumptive epidermis and the blastoderm will have an 

 epidermal margin in that region. 



In order to explain the fact that the whole of the blastodermal margin 

 in birds is ectodermal, we should have to postulate an expansion of the 

 presumptive epidermal area in addition to the flattening of the spherical 

 blastula. This makes the scheme more complex than that of Dalcq and 



