REPRODUCTION 



127 



AMPHIOXUS 



AMPHIBIAN 



Figure 6 7 Stages in cleavage and early gastrulation in eggs of chordates. A, Am- 

 phioxus (holoblastic cleavage, isolecithal egg with little yolk). B, Frog (holoblastic 

 cleavage moderately telolecithal egg with much yolk). C, Bird (meroblastic discoidal 

 cleavage, telolecithal egg with much yolk). D, Mammal (holoblastic cleavage, isolecithal 

 egg wi^h essentially no yolk). (From Storer and Usinger: General Zoology, 3rd Ed. Copy- 

 right 1957 by McGraw-Hill Book Co.) 



