80 THE MAMMALIAN EGG 



typified in the rat, mouse, hamster and guinea-pig, do not alter 

 appreciably during the development of the blastocyst and up to the 

 time of implantation. Generally, the zona pellucida remains un- 

 changed until shortly before implantation, though it was often 



Fig. 65 

 'Smoke rings' apparently binding together the two 

 sperm tails in 2-cell polyspermic rat eggs, (a) X 800; 

 (b) x 3,000. (From Austin and Bradem 1953b.) 



found to undergo some expansion in the hamster, with concomitant 

 increase in the size of the perivitelline space (Austin, iQ56d). On the 

 other hand, in the rabbit, ferret, dog (Fig. 66) and cat (Figs. 67 to 

 69), and in man and ape, the embryo expands some fifty- or hun- 

 dredfold in diameter, becoming strongly distended by the fluid that 

 accumulates in the blastocoele. Extreme forms of blastocyst are 

 found in the ungulates wherein it is a relatively enormous flaccid 

 spindle-shaped structure, containing little fluid. Form of blastocyst 

 is related to mode of implantation, which tends to be superficial 

 with the larger ones and interstitial with the smaller (see Amoroso, 

 1952). 



Studies have been made on the nature of the fluid in the rabbit 

 blastocyst, and these have shown that its composition differs in 



