STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 55 



differs in form and amount in different species; ejection occurs 

 chiefly about the time of fertilization and the first cleavage division. 

 Deutoplasmolysis has been described in the opossum Didclphys 



Fig. 46 

 Dog morula. X 250. (E. C. Amoroso.) 



(Hill, 1918; Hartman, 1919; McCrady, 1938), native cat Ddsyurus 

 (Hill, 1910), bat (Van der Stricht, 1909), guinea-pig (Lams, 1913), 

 cat (Van der Stricht, 1923; Hill and Tribe, 1924), pig (Heuser and 

 Streeter, 1929), ferret (Hamilton, 1934), horse (Hamilton and Day, 

 1945), field vole (Austin, 1957b) and rat (Odor, i960). In Didelphys, 

 the process takes an extreme form; the blastomeres of 2-cell and 

 4-cell eggs generally appear to have incomplete plasma-membrane 

 envelopes and the blastomere cytoplasm is in places continuous with 

 material that is eventually to be distinguished as discarded yolk. 



Fine structure. Few investigations have yet been made on the fine 

 structure of mammalian egg cytoplasm, the most detailed being 

 those of Yamada, Muta, Motomura and Koga (1957) on the mouse, 

 and Sotelo and Porter (1959) and Odor (i960) on the rat. In oocytes 

 and ootids, the endoplasmic reticulum appeared to exist only in the 

 form of a few small vesicles deficient in rna particles (ribosomes), 

 although Odor noted the presence of many atypical membranous 

 elements before the preovulatory changes. In the 2-cell egg, there 

 were many more such vesicles and occasionally they showed 

 continuity with the outer layer of the nuclear membrane, in a 

 maimer that has often been described in tissue cells. Sotelo and 

 Porter suggest that this difference in the 2-cell egg marks the 



