270 GERM-CELL CYCLE IN ANIMALS 



FIG. 73. Rhabditis nigrovenosa. Stages in maturation, fertilization, 

 and cleavage. A. Oogonium with twelve chromosomes. B. Sec- 

 ond maturation division. Pronucleus and second polar body each 

 with six chromosomes. C. Primary spermatocyte. D. Division of 

 primary spermatocyte. E. Second spermatocyte division ; one 

 chromosome delayed. F. Two spermatozoa each with six chromo- 

 somes. G. Cleavage spindle of egg showing two groups of chromo- 

 somes; one with six contributed by the egg, the other with five 

 contributed by the sperm. (After Schleip, 1911.) 



