STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 79 



Cytoplasmic cleavage can be inhibited or even, if it has not 

 advanced too far, reversed, whereupon a single cell is reformed with 

 a resting nucleus. The process is best known at the first division of 

 the egg and can be followed later by normal cleavage, the resulting 

 embryo now having twice the previous chromosome number. 

 Some forms of invertebrate parthenogenesis involve first-cleavage 

 inhibition as a means of regulation to diploidy. Inhibition after 

 fertilization results in tetraploidy. The few relevant observations 

 that have been made in mammals are set out clearly by Beatty (1957) 

 (see also Edwards, 1958a). 



The sizes of the blastomeres produced by the early cleavage 

 divisions are generally unequal, so that the morula in many animals 

 comes to be made up of larger and smaller cells which tend to 

 aggregate towards opposite poles (Fig. 44). The smaller cells are 

 destined to form the inner cell mass of the blastocyst and the larger 

 cells the trophoblast. Views concerning other distinguishing charac- 

 teristics of these two cell types have already been discussed (p. 61). 



In those animals in which the sperm tail enters the vitcllus at 

 fertilization (p. 69), the residue of this structure may, to judge from 

 studies on the rat, mouse and hamster, come to lie wholly within one 

 blastomere at the 2-cell stage, or be 'shared' by the two cells, passing 

 across from one to the other in the region of contact between them. 

 Similar distributions may be seen at later cleavage stages, though 

 the fate of the sperm tail becomes progressively more difficult to 

 determine, even in the rodent eggs, owing to its gradual dissolution. 



A small distinct dark circle of material seems to be accumulated 

 by the cleavage furrow in its inward movement and to persist for 

 a while after cleavage is completed. It rather resembles a smoke 

 ring, and may lie free in the cytoplasm of one of the blastomeres or 

 come to surround a sperm tail (Austin and Braden, 1953b) (Figs. 61b 

 and 62a). If, during microscopical examination, the sperm tail is 

 extruded from the egg by pressure on the coverglass, the 'smoke 

 ring' can still be seen surrounding the tail; it appears to have some 

 solidity. In those polyspermic 2-cell eggs in which both sperm tails 

 are shared between the two blastomeres, the 'smoke ring' may be 

 deposited around the tails and give the appearance of binding them 

 together (Fig. 65). 



The characteristic feature of the blastocyst is its thin-walled 

 bladder-like form, but wide variations on this basic pattern occur 

 among animals. The overall dimensions of the rodent embryo, as 



