REPRODUCTION 77 



mating most commonly occurred between 10 P.M. and i A.M., the average 

 interval between the onset of estrus and ovulation for the MacDowell-Bagg 

 albino strain at least is probably about 2 hours. The interval was found 

 to be quite variable, however, being certainly less than i hour in one case 

 and certainly more than 3 hours and 45 minutes in another. Ovulation 

 within 15 minutes of mating has been noted (81). Extensive data for the 

 rat (17) indicate that ovulation in the Wistar strain commonly occurs some 

 g hours after the onset of estrus, but may occur at least as early as 73^2 hours 

 and at least as late as 123^^ hours after the onset of estrus. It should be 

 remembered that the onset of estrus occurs much earlier in the evening in 

 this species than it does in the mouse. 



The rupture of all the mature follicles in an ovary seems usually to be 

 approximately synchronous (83), but there is evidence that an appreciable 

 interval may separate the individual ovulations in some cases (3, 81, 126). 

 Ovulation may not occur at every estrus, particularly in young virgin 

 females (3, 131). Conversely, estrus may not always accompany ovulation 

 (137). The mechanism of ovulation is not entirely understood, but there 

 is evidence that a thinning of the wall at the outer surface of the follicle and 

 an increase in internal fluid pressure both play a part (6, 131). 



Immediately after ovulation the eggs are found in the upper part of the 

 oviduct, presumably carried there by an outrush of follicular fluid at the 

 time of follicle rupture. The beating of the cilia of the infundibulum may 

 also help to carry them from the capsule into the oviduct. At the same 

 time the upper part of the oviduct becomes distended with fluid, a con- 

 dition easily seen in dissected animals under the microscope (83, 126, 127). 

 As the distension is not present prior to ovulation, it is a reliable sign that 

 ovulation has occurred (126). It has been stated that most, at least, of the 

 fluid is not derived from the follicles, but rather is secreted by the tubes 

 themselves (21, 83). 



Maturation and fertilization. — Maturation and fertilization of the egg 

 in the mouse and rat have been described by several authors (71, 74, 76, 84, 

 127). The following description is based on the work of Long and Mark 

 (84) except as otherwise noted. 



The whole maturation process requires not less than 4 nor more than 15 

 hours. At the onset of estrus the first maturation division is usually in 

 progress (126). Usually this division is completed, the first polar body 

 present, and the second maturation spindle already formed by the time 

 ovulation occurs. Occasionally, however, the egg is in the stage of the 

 first spindle or the first telophase at the time of ovulation, in which case 



