MATURATION 



159 



genetic eggs a second polar body is formed typically, leaving the reduced 

 number of univalent chromosomes in the egg nucleus, but then the 

 second polar body immediately reenters the egg, apparently taking the 

 place of an equivalent sperm nucleus and restoring the chromosomal 

 characters to the normal somatic condition, after which development 

 proceeds. The polar body need not be actually extruded from the egg 

 cell in order to give the same history, as long as the nuclear events are 

 equivalent (Fig. 84). In some eggs, even of species in which the history 

 is at times similar to that just described, a different method gives the 

 same result. Thus, while the second polar spindle may form typically, 

 the chromosomes upon it do not divide, and the equivalent of the second 



FIG. 84. Maturation in the parthenogenetic egg of the Echinoderm, Astro- 

 pecten. After O. Hertwig. A. First polar body formed but not extruded; 

 second polar division in early anaphase. B. First polar body extruded; second 

 polar division completed, the polar nucleus near the periphery. C, D, E. Stages 

 in the gradual approach and fusion of the second polar nucleus and egg pronu- 

 cleus, to form the cleavage nucleus. /. First polar body; //, second polar nu- 

 cleus; 9 , egg pronucleus. 



polar nucleus is never formed. The egg nucleus then re-forms with its 

 - chromosomes, but these are bivalent as shown by the character of 



the first maturation division, so that in effect the egg nucleus contains 

 the somatic number of chromosomes, which actually appears in subse- 

 quent divisions. It is therefore clear that while such parthenogenetic 

 eggs fail to receive a sperm nucleus they retain or receive back the 

 equivalent of such a nucleus in the form of the second polar body 

 nucleus, which is not lost as it is in eggs requiring to be fertilized. 



