M. C. CHANG 117 



lopian tubes of recipient rabbits, but no implantation was ob- 

 served after the transfer of 230 treated eggs. According to Chang 

 ( 1954 ) , the transformation of the whole group of chiomosomes in 

 the second maturation spindle into a "pronucleus" and the re- 

 sumption of mitotic division were also observed. The chromo- 

 somes of these blastocysts were determined to be diploid due to 

 the failure of abstriction of the second polar body. 



Although only a small number of artificially activated eggs de- 

 veloped into blastocysts, three definitely parthenogenetic young 

 rabbits were obtained by Pincus ( 1939 ) when 615 eggs were 

 subjected to various treatments and then transferred to recipient 

 does and "one egg in some 200 developed into a living partheno- 

 genetic rabbit" when freshly ovulated eggs were treated in situ 

 with ice water for 2 to 20 minutes (Pincus and Shapiro, 1940). 



From the above account, it is quite clear that the activation or 

 parthenogenetic cleavage of mammalian eggs without the pene- 

 tration of sperm is a widely spread phenomenon in vivo as well 

 as in vitro. Since most of the unfertilized eggs do not cleave 

 without the penetration of a spermatozoon, it must be assumed 

 that the cleavage of the unfertilized egg is only induced under 

 certain conditions. This is true also in the artificial activation 

 when eggs were subjected to an adverse environment for a short 

 time. In an abnormal environment, certain systems may be in- 

 jured or enhanced. As F. R. Lillie (1911) stated, "the nature of 

 the inhibition that causes the need of fertilization is a most funda- 

 mental problem." In this respect, the artificial activation of eggs 

 may be a release of the inhibition under certain circumstances. 



In his review of the general features of artificial partheno- 

 genesis, Tyler ( 1955 ) stated that "in general the percentage of 

 normal development is quite low even when a particular treat- 

 ment initiates development in all of the eggs in a manner indis- 

 tinguishable from that induced by sperm." This is attributable by 

 him to various factors, such as irregularities in distribution of 

 chromosomes, haploidy, lack of a proper division mechanism 

 and, in some instances, to failure to establish a plane of bilateral 

 symmetry. This does not explain why the activated eggs may 



