M. C. CHANG 121 



Pronuclear Behavior during Fertilization 



Cytological studies of fertilization in the mouse ( Sobotta, 1895; 

 Lams and Doorme, 1908; Gresson, 1941), in the rat (Tafani, 1889; 

 Sobotta and Burckhard, 1911), in the guinea pig (Rubaschkin, 

 1905; Lams, 1913), in the bat (Van der Stricht, 1910), in the 

 rabbit (Rein, 1883; Pincus, 1939), and in the ferret (Mainland, 

 1930) are well known. The cytochemistry of mammalian eggs 

 has been investigated recently by Dalcq (1954) and by Ishida 

 (1954). 



In the rabbit egg, the second polar body is formed about 45 

 minutes after sperm penetration and is succeeded by the forma- 

 tion of both pronuclei ( Pincus and Enzmann, 1932 ) . The pronu- 

 clear behavior and the correlation between male and female 

 elements at fertilization in the living rat egg were studied by 

 means of phase-contrast microscopy by Austin (1951b,c). He de- 

 scribed the change of the sperm head within 10 minutes after 

 penetration into the vitellus, the occurrence of the second matura- 

 tion division before any alteration of the sperm head, the move- 

 ment and the rotation of the second maturation spindle at telo- 

 phase, the change of the sperm head and that of the female 

 chromosomes into male and female pronucleus, and the three 

 stages of growth and coalescence of the nucleoli of both pronuclei. 

 In supporting the theory of protein synthesis put forward by 

 Caspersson ( 1947 ) , he suggested that the substance produced by 

 the nucleolus associated chromatin, after appropriate modifica- 

 tion by gene action and storage in the nucleoli, passes into the 

 cytoplasm during the terminal reduction of the pronuclei. 



As the whole process cannot be observed in vitro under the 

 same preparation, Austin (1951a) admitted that "it has been nec- 

 essary to depend upon eggs recovered from rats killed at suc- 

 cessively later times" in order to build up a picture of pronuclear 

 growth. To what extent these processes, especially the coales- 

 cence and division of nucleoli, are due to a degenerative 

 process is difficult to estimate. In the rabbit, eggs recovered at 

 various times after ovulation were stained with vital dyes and 



