302 



EVOLUTION AND GENETICS 



The cytoplasm of some eggs shows marked differentiation. 

 The eggs of Stijela -partita, for example, are reported by Conldin 

 as having an orange pigment distributed over the surface before 

 maturation, while during fertilization this pigment retreats to the 

 vegetal pole. Above the orange area there appears a layer of 

 clear cytoplasm and in the remainder of the egg gray yolk. Such 

 regional differentiation is sometimes associated with the initial 



B £ 



Fig. 174. — Diagram of zones of cytoplasmic differentiation and their distribu- 

 tion at the first cleavage. A, immature egg with no visible cytoplasmic 

 differentiation; B, mature egg with four zones; C, division of the egg; 

 D and E, two types of two-cell stages; D, Dentnlium or Styela, in which 

 one cytoplasmic zone passes completely into one cell; E, sea urchin or 

 Amphioxus type, with equal distribution of cytoplasm. (From Woodruff, 

 after Wilson.) 



cellular differentiation of the developing embryo but it has not 

 been shown to influence hereditary characters (Fig. 174). 



It is necessary to draw the conclusion that cytoplasm is merely 

 a material of construction. It is as essential in reproduction as 

 in individual life ])ut is apparently entirely under the control 

 of the nucleus, and particularly of the chromosomes. The two are 



