STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 83 



There is little really precise information on the cleavage rates of 

 mammalian eggs in vivo; this is owing to the difficulty of knowing 

 the exact time of ovulation, to the fact that fertilization may be 

 initiated at any time over a period of 12 hr after ovulation or even 

 longer, and, in polytocous animals, to the scatter in the time of 

 penetration of the eggs. In addition, it is reasonable to suppose that 

 eggs in any one species do not all develop at the same rate. Finally, 

 since the actual process of cleavage occurs relatively rapidly, direct 

 observation is rare, the time of cleavage has generally to be inferred 

 from the condition of eggs on recovery from the animal and a large 

 number of observations are necessary for even approximate esti- 

 mates. As a result, for animals of many species the figures so far 

 available from published reports show an extremely wide range of 

 variation and are almost valueless. Perhaps the most useful con- 

 clusions that can be drawn from this material are as follows : (a) The 

 best estimates are those for the rabbit ; this is largely because ovula- 

 tion is induced by coitus and is known to occur about 10 hr after 

 the stimulus. The most advanced eggs undergo the first cleavage at 

 about 12 hr after ovulation, the second at 16 hr, the third at 22 hr 

 and the fourth (becoming 16-celled) at 30 hr. The blastocoele is 

 first evident at about 60 hr and the main expansion of the blastocyst 

 takes place in the region of 90 hr (Lewis and Gregory, 1929a, b). 

 (b) The next most accurate estimates are those for some of the 

 laboratory rodents, owing to the large number of observations made 

 on them. Mouse eggs seem to develop quickest, the earliest be- 

 coming 2-celled at about 17 hr after ovulation, 4-celled at 38 hr, 

 and 8-celled at 47 hr. The blastocyst is recognizable at about 63 hr. 

 Clearly, the cleavage rate in the mouse is much slower than in the 

 rabbit; the impression that the mouse embryo 'catches up' at the 

 blastocyst stage is attributable to the fact that rodent blastocysts are 

 formed of many fewer cells than are rabbit blastocysts. Rat and 

 golden-hamster eggs cleave even more slowly, the earliest entering 

 the 2-cell stage at about 15 hr after ovulation, the 4-cell at 40 hr, 

 the 8-cell at 60 hr and the blastocyst at 80 hr. hi these three rodents, 

 sperm penetration commonly occurs 2 to 5 hr after ovulation, so 

 that fertilization can be said to require about 12 hr. (These figures 

 are based on the reports of Beatty, 1956a, who summarizes earlier 

 data on cleavage rates; Austin and Braden, 1954a; Braden and Austin, 

 1954b; Austin, I956d; Chang and Fernandez-Cano, 1958; and the 

 author's unpublished observations.) (c) From the data summar- 



