FERTILIZATION AND CLEAVAGE 93 



coitus (37 hours after ovulation) and the cavity of the 

 blastodermic vesicle may begin to form while the ova are 

 still in the tubes. Guinea pig (Squier, 1932) ova enter the 

 uterus in the 8-cell stage at the end of the 3d day after 

 copulation and the blastodermic vesicles form only in the 

 uterus at about 43^2 days after coitus. In the rat (Huber, 

 1915) the ova enter the uterus during the 4th day after 

 coitus in about 12 cells and start to form the blastodermic 

 vesicle during the 4th to 5th days post coitum, and in the 

 mouse (Enzmann, Saphir and Pincus, 1932; Lewis and 

 Wright, 1935) blastocyst formation occurs in the uterus 

 during the 4th day after copulation. 



The physiological factors governing the cleavage of mam- 

 malian ova have been scarcely examined. It has already 

 been stated that the whole course of cleavage of rabbit 

 eggs may proceed normally in vitro and in heterologous as 

 well as homologous blood plasma (Pincus, 1930). This 

 would seem to imply that no special environmental factors 

 supervene in the tubes. On the other hand the ova of mice, 

 rats and guinea pigs do not cleave under the ordinary 

 (or a variety of) tissue culture conditions. The reasons for 

 this species difference are not known though the superior 

 vitality of rabbit ova has been attributed to their unique 

 albumen coating; but Lewis and Hartman (1933) have over 

 a period of approximately 24 hours, observed the regular 

 cleavage in vitro of a monkey o\aim which lacks an albumen 

 coating. 



In the case of those ova which have not undergone cleav- 

 age in vitro one can only deduce that some limiting factor 

 obtaining in vivo has not been duplicated. Since it is known 

 that the secretory activity of the tubal epithelium is under 

 hormonal control of the ovary (c/. Snyder, 1923) it is pos- 

 sible that a special contribution to the economy of cleaving 

 ova is made by a hormonally induced secretion. The cleaving 

 ova of all mammals journey through the tubes during the 

 early life of the corpus luteum. The secretory activity of 

 the tubal epithelium changes markedly during the transi- 



