54 THE EGGS OF MAMMALS 



which Needham (1932) has designated ''an opening of doors'' 

 in the cell initiates the development of the ovum and makes 

 of a static cell one capable of transformation. What happens 

 subsequent to the activation process is often independent 

 of the process itself. The probability of cleavage and the 

 formation of a complete individual depends in part on the 

 nutritional environment and the chromosome constitution 

 of the activated egg. 



The activation process in non-mammalian ova has been 

 described in physico-chemical terms (see J. Loeb, 1913; 

 F. Lillie, 1919; Just, 1928; Runnstrom, 1933; Whit aker, 1933; 

 R. Lillie, 1934). There exists no similar information partic- 

 ularly for the ovarian eggs of mammals. The only estab- 

 lished index of an activation of ovarian eggs is the described 

 formation of the first polar body. It is conceivable that this 

 represents the first step in an activation process that would 

 go to completion if conditions were propitious. Perhaps the 

 same pituitary stimulus that induces polar body formation 

 might cause the formation of a cleavage spindle. The first 

 cleavage spindles observed by Branca (1925) may then be 

 considered the result of an activation process carried to 

 completion because adequate pituitary stimulation was avail- 

 able. On this basis the liberation of ova from the ovary 

 results in such a change of environment that the stimulus to 

 completion of activation is ordinarily no longer available. 

 Similarly mature ova retained in the ovary at the time of 

 ovulation ordinarily degenerate either because the proper 

 type of pituitary hormone is not active (c/. Hisaw's concep- 

 tion of the alternative action of follicle stimulating and 

 luteinizing hormones) or because of the partition of the 

 active hormone to other tissues {e.g., corpora lutea). 



We may consider two further alternative explanations 

 of the activation of ovarian eggs. It is possible that activa- 

 tion occurs in the ova of degenerating follicles because 

 (1) the breakdown of cells near the ovum results in the re- 

 lease of activating substances or (2) the initial stages of 

 atresia in the egg cytoplasm frequently involve structural 



