84 THE MAMMALIAN EGG 



ized by Amoroso, Griffiths and Hamilton (1942), it can be inferred 

 that the eggs of the goat, cow, sheep and pig pass from the 2-cell 

 stage to the 128-cell stage (six cleavages) in a mean time of about 

 112 hr (arriving at this point between 140 and 170 hr after coitus). 

 This represents a cleavage rate of about 19 hr per stage, an interval 

 of about the same duration as with rodent eggs. The blastocoele is 

 reported to be formed at about 5 days in the goat, 8 to 9 days in 

 the cow, 6 to 7 days in the sheep and 5 to 6 days in the pig (Beatty, 

 1956a). (Data on some other animals are given by Boyd and Hamil- 

 ton, 1952, and Beatty, 1956a.) 



The process of cleavage as thus far considered pertains- to meta- 

 therian and eutherian eggs. Cleavage in the prototherian (mono- 

 treme) egg is similar to that in other megalecithal eggs in that the 

 large mass of yolk is unaffected and even the cytoplasm does not 

 become divided into separate cells in the early stages. Cleavage 

 furrows divide the germinal disc into progressively smaller areas, 

 the cytoplasm in the deeper regions of each cell retaining continuity 

 with that of the other cells and with the yolk mass. Later, as the 

 number of cells increases, they do become separate units and form 

 a flattened blastodisc. With further cellular divisions, the blastodisc 

 comes to consist of several layers and a single layer of cells extends 

 out over the surface of the yolk. When the yolk is entirely covered, 

 the embryo is held to have reached the blastocyst state, though a 

 true blastocoele is apparently not represented. (For further details, 

 see Boyd and Hamilton, 1952.) 



Fragmentation of Eggs 



It has long been known that both ovarian oocytes and tubal eggs 

 are prone to undergo cytoplasmic division, apparently spontaneously 

 and often in a manner that superficially resembles normal cleavage. 

 The phenomenon has been described in a number of species : bat 

 (Van der Stricht, 1901), guinea-pig (Rubaschkin, 1906), armadillo 

 (Newman, 191 3), mouse (Kingery, 19 14), opossum (Hartman, 

 1919), water vole (Sansom, 1920), rabbit (Champy, 1923), rat (Mann, 

 1924), man (Krafka, 1939), ferret (Chang, 1950c, 1957b), hamster 

 (Skowron, 1956) and pig (Dziuk, i960). Though several authors 

 were attracted by the idea that parthenogenesis might on occasion 

 be displayed by mammalian eggs, the general conclusion was that 

 most if not all the instances of apparent cleavage were in fact caused 



