FOWL HKST DAY 



519 



a.p. 





a.o. 



77.5. 



B 



Kn;. 'J'.!"'. - Illustrating three stages of the blastoderm of the Fowl during the second half 

 of the first day of incubation. 



.", Mpai|iii' area ; <./>, pellucid an-a ; /'./>, head process; mes, Ixnuulary of sheet of raesodcrni ; 

 m./, medullary fold ; />.;/, 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) believt-s that an actual process of involution a reminiscence of 

 gastrulation by inva^inauon takes place. It must not be forgotten that any 

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



