M. C. CHANG 115 



ently observed after artificial activation by cold shock treatment. 

 In rabbit eggs, sliiinkage of the vitellus after sperm penetration 

 (Pincus and Enzmann, 1932) or after artificial activation (Thi- 

 bault, 1949 ) has also been reported. 



The formation of the second polar body may be a good criterion 

 of activation by spemi, but one must be sure it is the second polar 

 body, not the cleavage of the first polar body or a fragment of the 

 ooplasm. In the rabbit egg, it is fairly easy to distinguish between 

 the first and second polar body. The cliromosomes of the first one 

 are a group of rods and dots spread in the whole cytoplasm 

 whereas those of the second one are clumped together in a light 

 circular zone (Plate I, 2). The fragment of ooplasm usually has 

 no chromatin. 



In eggs of many species of marine invertebrates, it is practically 

 only the eggs that fail to extrude polar bodies upon artificial acti- 

 vation that are able to cleave and develop (Tyler, 1941). In the 

 rat egg, the formation of the second polar body is regularly ob- 

 served after artificial activation by cold treatment (Thibault, 

 1949 ) . This is also confirmed by Austin and Braden ( 1954a ) 

 based on their observation that the first polar body persisted only 

 in 1.3% of untreated eggs. In the rabbit egg, the second polar 

 body is rarely, if ever, observed after cold treatment in vivo or 

 in vitro (Thibault, 1949; Chang, 1954). 



According to Thibault (1949) eggs were rendered unferti- 

 lizable following artificial activation. In the rat egg, Austin and 

 Braden (1954b) reported that sperm can readily penetrate the 

 eggs that have emitted the second polar body after artificial ac- 

 tivation, and they came to the conclusion that "the development 

 of the block to polyspermy, the shrinkage of the vitellus, and the 

 emission of the second polar body are seen as independent proc- 

 esses, capable of being evoked separately. Only the block to 

 polyspermy appears to be a specific response to sperm penetra- 

 tion." Gynogenetic development comparable to that of frog eggs 

 treated with x-irradiated spermatozoa (Hertwig, 1913; Rugh, 

 1939) was not obseryed in rabbit eggs (Amoroso and Parkes, 

 1947). 



Cleavage of eggs in the ovary of guinea pigs to blastocyst stage 



