22 



EARLY EMBRYOLOGY OF THE CHICK 



grasps its fundamental similarity to the corresponding process 

 in forms with scanty yolk. In Amphioxus, gastrulation is an 

 inpocketing of the blastula (Fig. 6). A double layered cup is 

 formed from a single layered hollow sphere much as one might 



OASTRULATION IN FORM WITH ISOLECITHAL EGG HAVING ALMOST NO YOLK AMPHIOXUS. 



GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING MODERATE 

 AMOUNT OF YOLK-AMPHIBIA. 



hip. 



gastrocoele 



OASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING LARGE 

 AMOUNT OF YOLK BIRDS. 



Pic. 6. Schematic diagrams to show the effect of yolk on gastrulation. 

 Abbreviations: blc., blastoccele; bid., blastoderm; blp., blastopore; ect., ectoderm; 

 nt., cntoderm; mit., cell undergoing mitosis; yk., yolk: vk.jr.. yolk srranules- 

 yk.p., yolk plug. 



push in a hollow rubber ball with the thumb. The new cavity 

 in the double walled cup is termed the gastrocoele. The open- 

 ing In.ni I IK outside into the gastrocoele is called the blastopore. 



