88 THE EGGS OF MAMMALS 



in the first third of the oviduct appear cytologically normal 

 (Mann, 1924). According to the data of Long and Evans 

 (1922) the ova remain in this portion of the oviduct for 

 about 33 hours. Hartman's (1932a) data on timed matings 

 in Macacus show that fertile matings occur only between 

 the 9th and 18th days of the menstrual cycle with maximum 

 between days 11 and 16. This, of course, does not imply 

 that the ova are fertilizable for several days, but presumably 

 that ovulation may occur at any time during the critical 

 9 day period. Matings time in relation to the onset of 

 oestrus in the sheep (Quinlan, Mare and Roux, 1932) and 

 the pig (Lewis, 1911) indicate a maximum period of fertility 

 of 48 hours. 



It is unnecessary in this monograph to discuss the cyto- 

 logical details of fertilization and cleavage in mammalian 

 ova, since these are now textbook commonplaces. Our inter- 

 est is primarily in the physiological mechanisms underlying 

 these events and their relation to the dynamics of growth 

 and development. We shall again discuss certain aspects 

 of the fertilization process in the chapter dealing with the 

 activation of unfertilized eggs. Now we shall turn our 

 attention to the relatively scant data that deal with the 

 mechanism of cleavage in tubal ova. 



Until fairly recently no very accurate data on the rate 

 of cleavage in tubal ova have been available. This has been 

 due in part to the difficulty of timing ovulation. Even now 

 it is possible to construct only approximate growth curves 

 for a limited number of species. These curves are presented 

 in Figure 29. It will be noted that rabbit ova cleave much 

 more rapidly than those of the other species (see Plate VII). 

 It is a matter of some interest to ascertain whether this 

 difference in the cleavage rate is the result of an especially 

 stimulating tubal environment in rabbits, or whether the 

 cleavage rate is an inherent property of the ova. The data 

 on the monkey were, in fact, deduced from Lewis and 

 Hartman's (1933) observations of cleavage in vitro, and 

 may be taken to indicate that segregation from the tubes 



