THE ROLE OF THE CELL ORGANS IN HEREDITY 



329 



the egg is virtually isotropic, and through several succeeding cell gene- 

 rations the blastomeres are equipotential, thai is, equally capable of 



developing into any part of the body or even into the whole of it. Thus 

 the embryonic parts, and hence many of the individual's characters, are 

 not definitely marked out until a comparatively late stage. On the 

 contrary, there are forms in which the axis of polarity and certain funda- 

 mental embryonic parts are roughly delimited in the egg cytoplasm in 

 such a way that an alteration in the relative positions of the egg materials 

 brings about a corresponding alteration in the character of the resulting 

 individual. As an illustration of such internal differentiation may !»<• 

 taken the case of Styela, an ascidian, described by Conklin (1915 In 

 the egg of Styela there are four or five distinct kinds of plasma arranged 

 in a definite order and distributed in a regular manner as cleavage pro- 

 ceeds, each kind eventually giving rise to a certain portion of the embryo. 



CTENOPHORE. 



TUKBELLATIIAN ECHINODERn A5CIDIAN 



Fig. 128. — Eggs of various animals, showing the patterns assumed by the materials 

 which give rise to the various body regions. In the first three the egg has undergone 

 division, and the plasmas becoming ectoderm, mesoderm, and endoderm are represented 



in clear white, cross-hatching, and parallel ruling respectively. In the fourth egg two 

 divisions have occurred, and several definitely arranged substances are distinguishable 

 (After Conklin, 1915.) 



Substances which are yellow, gray, slate-blue, and colorless give rise 

 respectively to muscle and mesoderm, nervous system and notocord 

 endoderm, and ectoderm. "Thus within a few minutes after the fertiliza- 

 rion of the egg, and before or immediately after the first cleavage, the 

 anterior and posterior, dorsal and ventral, right and left poles arc clearly 

 distinguishable, and the substances which will give rise to ectoderm, 

 endoderm, mesoderm, muscles, notocord and nervous system are plainly 

 visible in their characteristic positions" (Conklin 191"), p. lis . If 

 such eggs arc placed in a centrifuge the various substances may be 

 made to assume an entirely abnormal stratified arrangement which 

 in turn "may lead to a marked dislocation of organs; the animal may be 

 turned inside out, having the endoderm on the outside and its skin and 

 ectoderm on the inside, etc." (p. 321). Such a behavior emphasizes 

 the determinative character of the cytoplasmic pattern clearly present 

 in many eggs. It has further been noted that the eggs of various animal 

 phyla are characterized by distinct patterns in the arrangmeent of their 

 visibly different materials (Fig. 128). "The polarity, symmetry and 



