154 



GENERAL EMBRYOLOGY 



A 



FIG. 80. Diagrams representing the behavior of the chromosomes during 

 fertilization and maturation. The differentiation of the three kinds of chromo- 

 somes is indicated by the number of small circles in each. d\ chromosomes 

 derived from the spermatozoon (sperm pronucleus) (black circles) ; ? , those from 

 the egg (egg pronucleus) (white circles). The somatic number of chromosomes 

 is six. A. Entrance of spermatozoon. B. Fusion of egg and sperm pronuclei, 

 forming the first cleavage nucleus. C. Splitting of chromosomes in equatorial 

 plate, during the division of any somatic, oogonial, or spermatogonial cell. D. 

 Primary oocyte or spermatocyte in synapsis (telosynapsis). Fusion of similar 

 chromosomes of maternal and paternal origin. E. Longitudinal splitting of 

 bivalent chromosomes during first maturation division. F. First division 

 completed forming the two secondary oocytes or spermatocytes. The nuclei 

 are alike in composition. G. Transverse division of chromosomes during second 

 maturation division (reducing division postreduction). H. The resulting 

 four cells. With respect to each chromosome the cells are of two kinds, numer- 

 ically equal. 



