108 THE EGGS OF MAMMALS 



in these ova compares favorably with those observed in 

 invertebrate ova, with the possible exception of the sea- 

 urchin eggs, a very large proportion of which (as much as 

 100 per cent) may develop regularly into swinaming larvae 

 (Hindle, 1910; Loeb, 1913). 



It was not possible to make any extensive cytological 

 study of the ova described. The few sectioned and stained 

 eggs obtained, indicate that in ova segmenting regularly 

 the nuclei and cytoplasm are normal in appearance. In 

 ova segmenting irregularly the situation is apparently rather 

 comphcated. There are obvious evidences of degeneration. 

 Some cells contain nuclei, others do not, and the cytoplasm 

 is often quite degenerate. One observes ova with several 

 nuclei and no distinct cell divisions. In the case of one fairly 

 regular ovum there were at least 37 chromosomes in an 

 incomplete metaphase plate. 



Upon consideration of the various factors involved in 

 the technique of explanting the ova it seemed most likely 

 that those young ova which underwent a normal partheno- 

 genetic cleavage were stimulated by a gradually developed 

 hypertonicity of the culture medium. For in these experi- 

 ments the ova were cultured in watch glasses in a moist 

 chamber, where the evaporation of a small amount of water 

 from the plasma culture was possible. If this conclusion 

 is true then at least one of the many types of parthenogenetic 

 stimuli known to be effective with non-mammalian ova is 

 similarly stimulating to mammalian eggs. 



In order to examine this question further the writer and 

 Dr. E. V. Enzmann (Pincus and Enzmann, 1936a) have 

 studied the effect of known methods of parthenogenetic 

 stimulation upon rabbit ova. We took as our criterion of 

 activation the production of the second polar body, which, 

 as we have seen in the experiments with semination in vitro, 

 is entirely adequate. 



The data of these experiments are given in Table XIX. 

 They demonstrate that short treatment with solutions of 

 relatively low hypertonicity are certainly effective in in- 



