OVARIAN ATRESIA AND PARTHENOGENESIS 59 



where alterations in normal hormone balance occur which 

 are sufficient to cause a preponderating activation stimulus. 

 Such may in fact be the basic cause of certain undoubtedly 

 normal early development in ovarian eggs reported by a 

 number of observers. In Plate IV, Figures 1 to 3 and Fig- 

 ure 1, Plate V, are presented various stages of pre-cleavage 

 development found in ovarian eggs. The multinucleate con- 

 dition of the egg of Figure 4 may be due to chromatin 

 fragmentation, but the cleavages of the eggs of Figures 5 

 and 6 of Plate IV and Figures 2 and 3 of plate V are com- 

 pletely normal. It seems clear that at any one of these stages 

 definite atresia of the ovum may set in, preventing further 

 development. Similar arrests of development may occur in 

 parthenogenetically activated invertebrate ova if the acti- 

 vating treatment is not carefully controlled (c/. Loeb, 1913; 

 Just, 1928). Entrance into the cleavage process is likewise 

 dependent upon a rather nice balance of developmental 

 events. Furthermore, the processes involved in cleavage 

 may indeed be independent of the activation process. 

 Runnstrom (1933) has shown that sea-urchin eggs poisoned 

 by monoiodoacetic acid can be fertilized but that segmenta- 

 tion soon ceases and ordinarily just before the dissolution 

 of the nuclear membrane of the first cleavage division. 



In later chapters we shall discuss further the problems 

 involved in parthenogenetic activation. Now it is sufficient 

 to indicate that there is a probability of activation of ovarian 

 eggs but that a complete activation is dependent upon a 

 balance of events which must presumably be rarely attained 

 in the ovary. Even if the activation reaction proper occurs 

 and segmentation ensues the probabilities that post-cleavage 

 stages will be entered are made extremely small not merely 

 because of the physical limitations imposed by the structure 

 of the ovary, but because, as we shall demonstrate later 

 (Chapter IX), the growth stage of the embryo is entered 

 into only as the result of a definite hormonal stimulus during 

 the luteal phase, and conversely is definitely inhibited by 

 oestrin. It is therefore surprising that the blastula and 



