130 THE EGGS OF MAMMALS 



itself is under the control of a cyanide-labile system. The 

 process of cleavage is apparently independent of the activity 

 of the primary sex hormones, oestrin and progestin. 



Tubal rabbit ova readily exhibit parthenogenetic cleav- 

 ages under certain conditions of explantation in vitro. Parthe- 

 nogenetic activation can be initiated experimentally by treat- 

 ment with cytolytic agents, by exposure to hypertonic 

 solutions, and by heat treatment. 



The development of the blastodermic vesicle in vivo is 

 conditioned by the activity of corpus luteum secretions. In 

 the absence of the corpus luteum development does not 

 occur beyond that stage in which ova just entering the uterus 

 are found. The evidence indicates that the corpus luteum 

 secretions either stimulate the eggs directly or provide 

 through stimulation of the uterine endometrium a suitable 

 environment for the developing blastocysts. Oestrin and 

 allied compounds prevent blastocyst growth by inhibiting 

 the corpus luteum effect, the ova being most sensitive to 

 this inhibition during the early blastocyst stages. 



The implantation process itself is also under hormonal 

 control. In the rat and mouse ovum implantation is delayed 

 during lactation. This delay appears to be due to excessive 

 corpus luteum secretion. 



The development of various techniques for the explanta- 

 tion of ova both in vivo and in vitro makes available a variety 

 of experimental investigations of the manmaalian ovum. 

 The ova of certain forms are particularly adapted to experi- 

 mental manipulation. Mammalian ova normally develop in 

 a homeostatic environment. Certain components of this 

 homeostasis sharply limit the extent and nature of ovum 

 development at certain stages. During other phases of its 

 growth the ovum appears to be a relatively independent 

 organism. Careful investigation of the physiological proc- 

 esses occurring in the ovum itself and in its homeostatic 

 environment is made possible by the various explantation 

 and transplantation techniques. 



