REPRODUCTION AND LIFE CYCLES 



421 



has the ventral body wall completed early in development. In such 

 forms the yolk that remains is carried within the body of the embryo 

 and is accessible as fuel for further metabolism. 



ectoderm-*^ 



.hldi 

 *ilastocoele^ 



IbU. isoWcifhal 



ed'tf . amphioxa? 



f "blastopore 



fiCtocCarm 

 endodarm 



"blC. 





telol<2cLthal 

 egig. amphibicin 



ectoderm ^Wc. 



-blp. 

 endode.] 



giastrocoelQ 



teldleciti2al 

 eg"g - toiT'd 



Diagram showing effect of yolk on the formation of the gastrula. Read text p. 420 

 and attempt to describe the etfect of yolk on gaslrulation. (After Patten.) 



In many of the fishes that are relatively large-yolked forms, develop- 

 ment is similar. Young fry of the small-mouthed bass carry around 

 enough yolk to maintain their " flame of life " for about two weeks, 

 after which they begin feedhig on the usually plentiful plankton 

 organisms. Whereas in a macrolecithal type with an abundant 

 supply of yolk, such as a bird's egg, the gut fails to close until a 

 much later date, the embryo literally floating on top of the mass of 

 potential food. Even as development continues there is such a vast 

 quantity of yolk present that it appears impossible for the embryo 

 to complete the ventral body wall until much of the potential food 

 material has been absorbed. As this process takes some time the 

 embryo remains independent of other sources of food material until it 

 gradually depletes the supply, and surrounds the remainder of the 

 yolk with the continued outgrowth of the gradually extending germ 

 layers. 



