244 



PRINCIPLES OF EMBRYOLOGY 



blastoderm but the presumptive map differs radically from that of the fish 

 in that the edge of the blastoderm is in this case constituted of ectoderm. 

 The presumptive mesoderm lies inside the margin and when the endoderm 

 occurs on the surface, as it does in certain reptiles, it is the most centrally 

 placed area of all. As between reptiles, birds and mammals, there are 

 obviously great similarities, such differences as are found being dependent 

 on the extent to which the endoderm is formed by delamination or is 



AMPHIBIA 



blastopore 



TELEOSTS 



REPTILES AND BIRDS 



Figure ii.io 



Maps of presumptive areas. The large circles in the teleost and reptile maps 

 represent yoUc; other conventions as before. In the amphibian map, A 

 represents the point at which the yolk would have to be inserted to give the 

 teleost map, and B that required to give the reptile-bird map. (From Wad- 



dington 1952.) 



originally at the surface, and on the degree to which the site of mesoderm 

 invagination is drawn out into a primitive streak or concentrated into a 

 chorda mesodermal canal. 



The movements in the eggs of all the groups are rather similar in general 

 type. There are movements of expansion in all directions, affecting parti- 

 cularly the ectoderm. There is a 'dorsal convergence', in which tissues be- 

 come narrower from side to side and simultaneously elongated, and do this 

 the more intensely the nearer they lie to the dorsal plane. Again, there are 

 tendencies for invagination, that is, for certain tissues to plunge inwards 

 from the surface and continue their movements at a lower level. Finally, 



