VESICLE GROWTH AND IMPLANTATION 121 



minimum daily sterilizing dosage for days 3 and 4 is 150 rat 

 units of oestrone-in-oil. When lesser dosages are injected a 

 partially sterilizing effect is observed. This partially steriUz- 

 ing effect is measured by observing the ratio between the num- 

 ber of corpora lutea and the number of implantations. The re- 

 lation of the implantation ratio to the hormone dosage is given 

 in Figure 32. It will be seen that even relatively low hormone 

 dosages have a lethal effect upon a number of the embryos. 

 This effect may be due either to prevention of implantation 

 of vesicles developing normally till implantation time or to a 



120 150 



Fig. 32. Abscissa: oestrone dosage in R.U. per day. Ordinates: A, per cent 

 of embryos unimplanted; B, number of unimplanted embryos per female. 

 (From the American Journal of Physiology.) 



degeneration before implantation. The latter alternative 

 seems most likely when one observes the degenerated condi- 

 tion of the preimplantation blastocysts. In addition prac- 

 tically all the embryos that do become implanted are normal 

 in appearance, and, in fact, give rise to normal young at 

 term (rabbit no. 47). 



W^en eggs in the blastocyst stage are placed in culture 

 they will develop normally for 24 to 36 hours (Brachet, 1913; 

 Pincus, 1930). Cleaving ova will, as we have seen, develop 

 for several days and collapse when the blastocyst stage is 

 reached and presomite stages continue development for 

 3 to 9 days. This implies that the explanted blastocyst 

 either carries with it from the uterine environment a limited 

 supply of necessary nutrition or that it rapidly exhausts 

 the necessary materials from the ordinary culture medium. 



