EARLY DEVELOPMENT OF THE CHICK. 



73 



The processes of development thus far described go on while the egg 

 is traversing the oviduct. Development ceases when the egg is laid and cools; 

 it begins again only if the temperature is raised. If the temperature remains 

 below about 25 (Centigrade) there is no appreciable development, but if 

 brought up to about 38, which is the optimum, development progresses 

 normally. And from now on, the ages of embryos are reckoned from the 



Area opaca 

 Area pellucida 



Primitive streak 



~ Area pellucida 



f- Area opaca 



T~ Primitive streak 



Blastopore 



(crescentic groove) 



FIG. 44. Surface views of blastoderms of Haliplana, showing formation of primitive streak. 



Schauinsland. 



beginning of incubation; not from the time the egg is laid nor from the time 

 cleavage begins. 



Gastrulation in the bird seems to be a simple process as compared with 

 that in the frog; in some respects it is even simpler than in Amphioxus. 

 Rapid cell proliferation is of course a common incident in all three cases, 

 particularly along the lip of the blastopore. In Amphioxus invagination 

 plays the important part; involution and epiboly are less prominent. In the 



Area opaca 

 Area pellucida 

 Head process 



Hensen's node 



Primitive streak 



Primitive groove 



Post, lip of 

 blastopore 



FIG. 45. Surface view of embryonic disk of chick. Bonnet. 



fro'g invagination is greatly reduced, while involution and epiboly are the 

 most conspicuous features. In the bird invagination and epiboly can scarcely 

 be said to occur at all; involution appears to be the essential process, and 

 with it a specially marked migration of entodermal cells beneath 

 the ectoderm. If an immediate cause for the differences in the three forms 

 is sought, the yolk content of the egg offers itself as a mechanical influence 

 which must be accepted as a most important factor. 



