ONTOGENESIS 



157 



FIG. 61. Diagrams illustrating the maturation, zygosis, and cleav- 

 age of an animal egg (somatic number of chromosomes, 4) ac, normal 

 mitosis of oogonium ; d e, tetrads and formation of first polar body (p. 6 

 1), in one of the cells of c ; fh, formation of second polar body, division 

 of the first and entrance of sperm ; ij, fusion of egg and sperm nuclei, 

 each with reduced number of chromosomes ; k I, first cleavage of zygote. 



its cell genealogy, so to speak back to the unicel- 

 lular zygote, and since the chromatin of the nucleus 

 of the zygote is derived, half from one gamete and 



