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GENERAL EMBRYOLOGY 



FIG. 140. History of the paternal chromatin during the first cleavage in the 

 pseudohybrid sea-urchin, Sphcerechinus ? X Strongylocentrotus d\ After Herbst. 

 A. First cleavage figure. Sperm and egg pronuclei associated, but not fused. 

 Chromosomes beginning to form in the egg pronucleus. B. Chromosomes in 

 both pronuclei; in separate groups with separate spindles. C. Anaphase of 

 first cleavage. Maternal chromosomes reaching the poles. Paternal chromatin 

 (chromosomes no longer) forming an irregular mass, spun out on the spindle 

 between the maternal chromosome groups. D. Division completed. Daughter 

 nuclei reconstructed and consisting entirely of maternal chromatin. One of 

 the cells contains a small vesicle consisting of the paternal chromatin, which takes 

 no further share in cleavage. <?, chromatin of the sperm pronucleus. 



