22 



EARLY EMBRYOLOGY OF THE CHICK 



J 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 



GASTRULATION IN FORM WITH ISOLECITHAL EGG HAVING ALMOST NO YOLK— AMPHIOXUS. 



GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING MODERATE 

 AMOUNT OF YOLK— AMPHIBIA. 



gastrocoele 





GASTRULATION IN FORM WITH TELOLECITHAL EGG CONTAINING LARGEj 

 AMOUNT OF YOLK-^BIRDS. 



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

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

 ent., entoderm; mit., cell undergoing mitosis; yk., yolk; yk.g., yolk granules; 

 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 from the outside into the gastrocoele is called the blastopore. 



