78 BIOLOGY OF THE LABORATORY MOUSE 



first polar body formation is completed very shortly thereafter. The polar 

 body is quite large. Its future history is variable; it may degenerate while 

 the egg is in the one cell stage, or persist as late as the morula stage. Occa- 

 sionally it divides in two (8i). Where mating occurs at the onset of estrus, 

 sperm are probably usually present in the upper end of the tube at the time 

 of ovulation (126). Each egg is surrounded by a zona pellucida and, out- 

 side this, a covering of cumulus cells. The cumulus cells are sticky, and 

 all the eggs in one tube usually are massed into a clump. The sperm 

 penetrate these coverings quite rapidly, partly dispersing the cumulus cells 

 in the process, perhaps by enzyme action (108), and reach the vitellus in less 

 than 2 hours (81). The penetration of the vitellus may be regarded, by 

 definition, as the actual moment of fertilization. At the time it occurs the 

 second maturation spindle is invariably present. In the absence of fertili- 

 zation, the second polar body does not form; where fertilization occurs, 

 second polar body formation ensues rapidly (71, 81, 84), and the processes 

 of normal development are initiated. 



Since estrus lasts for some 12 hours in mice and rats, mating may 

 occur several hours after ovulation, the eggs meantime lying unfertilized 

 in the oviducts. For several hours they retain their capacity for normal 

 fertilization and development, but in a relatively short time degenerative 

 processes make their appearance (13, 108). 



The transport of sperm and eggs. — Sperm reach the upper end of the 

 uterus in the rat almost at once after mating (53, 58, 64, 1 14). Throughout 

 heat the uterus is distended with fluid, and transport of sperm to the mouth 

 of the oviduct is accomplished not by the sperm's own motility but as a 

 result of a churning action of the uterine wall acting on this fluid. Trans- 

 port of sperm through the oviduct is somewhat slower, but Lewis and 

 Wright (81) find that they may reach the ovarian end of the oviduct, where 

 fertilization occurs, within 15 minutes of mating. The mechanism involved 

 in this transport of the sperm toward the ovary, as also in the abovarian 

 transport of the fertilized eggs, is somewhat obscure, though a churning 

 action may again be involved in the sperm transport. The subject has 

 been thoroughly discussed by Parker (106) and Hartman (in Allen, 6), and 

 the interested reader is referred to these authorities. 



The spermatozoa of the mouse retain their fertilizing ability in the ovi- 

 duct for about 6 hours; their motility ceases only after 133^2 hours. Their 

 period of survival in the uterus is shorter than in the oviduct (93). 



Pseudopregnancy. — Sterile matings in the mouse and rat induce a 

 condition called pseudopregnancy, characterized by a delay of the next 



