98 



Vertebrate Embryology 



side of the egg ; and segmentation is confined 

 to this small area. We have, then, these four 

 main methods of segmentation : the complete, 

 which may be equal or unequal, and the par- 

 tial, which may be superficial or discoidal. 



~W 



FiG. 35. — Surface view of the blastoderm 



AT A LATER STAGE OF SEGMENTATION THAN THAT 



SHOWN IN FIG. 34 B. Also somewhat more 

 ENLARGED. (After Duval.) 



It may be well, at this point, to give a brief 

 description of the relation between the amount 

 and distribution of the food yolk, and the 

 phenomena of segmentation and gastrulation. 



Total cleavage occurs in eggs which, as a 

 rule, are small, and which contain a small or 

 moderate amount of yolk. If this small 



