FOWL— FIRST DAY 



519 



Fig. 225. — Illustrating three stages of the blastoderm of the Fowl during the second half 

 of the first day of incubation. 



a.o, opaque area ; a.p, pellucid area ; ll.p, head process ; rues, boundary of sheet of mesoderm ; 

 m.f, medullary fold ; p.g, primitive groove ; p.s, primitive streak. 



forwards and outwards, and eventually is continuous all round with 

 the thickened marginal part of the blastoderm. 1 



1 This thickening of the posterior edge -of the blastoderm presents in sagittal 

 section a striking resemblance to a gastrular lip growing back over the yolk and 

 Patterson (1907) believes that an actual process of involution — a reminiscence of 

 gastrulation by invagination — takes place. It must not be forgotten that any 

 explanation of such obscure developmental phenomena in Birds must, to be reliable, 



