THE DEVELOPMENT OF THE INDIVIDUAL 



527 



Animal 

 pole 



Gray crescent Vegetal pole 



B G 



Blastocoel 



Yolk cells 



Enteron 



E 



G 



Gill arches 



Yolk pi 

 Blastopore 



Endoderm 



Ectoderm 

 Blcl. 



Neural groove 



jm 



Notochord 

 Neurenteric 

 canal 



Yolk plug 



Spinal cord 



Midbrain 

 Forebrain 



Anal 

 region 



Rectum' \ \& 

 Yolk 

 Ectoderm 



Enteron 

 \- Sucker plate 

 Endoderm 

 Mesoderm 



j ii,ctoQerm' j£ 



Foundations of Biology, Woodruff, The Macmillan Company 

 Fig. 33.3. Early embryology of the frog. Note the unequal type of total cleavage 

 due to the concentration of yolk at the vegetal pole. 



pore, but it is not a pore at all because it is filled with the yolk-bearing 

 cells which form the yolk plug. 



In some chordates the amount of yolk in the zygote is so great that 

 there can be no total cleavage, but rather there is partial cleavage which 

 does not divide the entire zygote. This type of zygote is found in the 

 higher fishes, birds, and reptiles. If we consider the fertilized hen's egg 

 as an example, the part which we call the "yolk" is the true zygote. 



