78 THE MAMMALIAN EGG 



spermatozoon is said to enter the ovarian follicle and initiate fertiliza- 

 tion there, and so the eggs emit both the polar bodies before leaving 

 the follicle (Bluntschli, 1938; Strauss, 1938, 1950). The same rela- 

 tions may hold also for the shrew Blarina hrevicorda (Pearson, 1944). 

 The eggs of the dog, fox and possibly the horse are ovulated as 

 primary oocytes and must produce both polar bodies after reaching 

 the Fallopian tube (Van der Stricht, 1923 ; Pearson and Enders, 

 1943; Hamilton and Day, 1945). In the dog, sperm penetration 

 occurs early, sometimes whilst the egg still has a germinal vesicle, 

 but the formation of the male pronucleus does not begin until the 

 second meiotic division is in progress. In the fox, on the other hand, 

 sperm penetration is delayed until after the formation of the first 

 polar body. Some details of time relations are given by Austin and 

 Walton (i960). 



Cleavage of the Fertilized Egg 



As the first cleavage mitosis reaches telophase, the vitellus of the 

 egg elongates, the surface dips in around the lesser circumference 

 and the constriction continues until the egg is divided into two 

 blastomeres, within each of which a resting nucleus becomes con- 

 stituted. The plane of cleavage is said to follow a line passing 

 through the positions formerly occupied by the centres of the two 

 pronuclei as they lay at syngamy (Van der Stricht, 1923). Division 

 of the blastomeres of the 2-cell egg is seldom synchronous, so that 

 a 3 -cell stage is normally interposed between the 2-cell and 4-cell 

 stages. Similarly, though the stages of eight cells, sixteen cells, 

 thirty-two cells and so on are customarily mentioned as representa- 

 tive of steps in embryonic development, and are in fact most often 

 met with, all the intermediate cell numbers are also seen. With each 

 successive stage of cleavage, the size of the blastomeres is roughly 

 halved, until it reaches about that of most of the tissue cells in the 

 organism concerned. During cleavage, the total mass of cytoplasm 

 actually decreases, presumably because yolk materials are used up 

 to provide energy for the maintenance and division of the cells. 

 The diminution in cytoplasmic volume from the i-cell stage to the 

 8-cell stage has been found to be about 20 per cent in the cow, 

 40 per cent in the sheep, 30 per cent in the ferret and 25 per cent in 

 the mouse (see Hamilton and Laing, 1946). Cell divisions subsequent 

 to the cleavage phase are associated with increase in size (growth) of 

 the embryo and with intake of nutrients by the embryo. 



