SUMMARY AND RECAPITULATION 129 



Pituitary hormones are definitely concerned in the final 

 stage of ovum maturation, the first polar division which 

 normally occurs in the ovary of most mammals. The pi- 

 tuitary secretions do not affect the eggs directly but initiate 

 changes in the follicles which make for maturation in the ova. 

 Similar changes occur in atretic follicles with a resulting 

 "pseudomaturation" in the ova of such follicles. The initia- 

 tion of ovum activation represented by the first maturation 

 division occurs in vitro simply upon the explantation of 

 ovarian eggs. Maturation in vivo and in vitro can be ex- 

 plained as the result of a functional isolation of the ovum 

 from the follicular epithelium. It is held probable therefore 

 that the parthenogenetic development of ova observed in 

 mammalian ovaries occurs as the result of the establishment 

 in the follicle of special activating conditions. 



Parthenogenetic development of unfertilized tubal ova 

 rarely if ever occurs in vivo. In most eutherian mammals the 

 eggs are shed surrounded by follicle cells. If sperm are not 

 present the surrounding cells slowly fall away, and the 

 naked ova descend into the lower portion of the tubes 

 where they degenerate and are eventually either resorbed 

 or washed out into the uterus. When sperm are present 

 there is a rapid dissolution of the surrounding follicle cells 

 due to the action of a heat labile substance carried by the 

 sperm. It has been claimed that this same substance acti- 

 vates the ova into forming the second polar body, but the 

 available evidence is contradictory. Tubal eggs remain 

 fertilizable for a few hours in the rabbit, and for thirty 

 hours in the ferret. 



Manamalian ova may be fertilized in vitro and normal 

 cleavage ensues. This is most readily demonstrated with 

 rabbit ova, for the ova of most of the other forms examined 

 do not cleave or develop appreciably under the ordinary 

 conditions of tissue culture. Segmentation in vivo occurs at 

 fairly characteristic rates in the various species of mammals. 

 The cleavage rate in rabbits is definitely correlated with the 

 adult size of the strain employed. The cleavage process 



