362 



THE NF.W INDIVIDUAL 

 Table 1 9.1 



Part IV 



Arrangements of Yolk and the Accompanying Types of Cleavage 



IN the Embryo 



an interval varying with the kind of animal and environment but often very 

 soon. 



The Substance of the Embryo Arranged in the Egg. In certain kinds of eggs 

 there are special regions in which pigment is present or absent, or yolk is 

 sparse or abundant. Either by following these visibly pigmented zones or 

 coloring them with vital dyes they have been traced to particular destinations 

 in the embryo. In the fertilized egg, these future organ regions are more defi- 

 nite than before fertilization. The substance of the egg takes part in an active 

 organization for the future development of the embryo. In the unfertilized egg 

 of the tunicate, Styela (Cynthia), one of the lower chordates, orange pigment 

 is uniformly distributed through the cell. But by streaming movements of the 

 cytoplasm during fertilization it is later concentrated into a yellow crescent 

 that marks the future posterior end of the embryo (Fig. 19.3). On the oppo- 

 site side of the egg is the gray crescent that becomes its anterior end. During 

 early development, the protoplasm of the yellow crescent is distributed to 

 form the middle layers of cells or mesoderm; the gray crescent becomes noto- 



