BIOLOGY OF EGGS AND IMPLANTATION 



839 



\\ ^ 



N 



SF 



6 



Fig. 11.12. \'nri()U.< ,-l;i-iv.s in the traii.-loiiuat ion ol I lie Ina.l nl iIm I. i i ili/iii- -|m i m leading 

 to the formation of the male pronucleus. Note loss of contrast of the liead as it enlarges. The 

 changes in the head from 1 through 6 require 2 to 3 hours. Observations were made on the 

 living egg, in vitro, and examined with phase contrast objectives. P, perforatorium; N, sperm 

 nucleus; SF, sperm flagellum (Austin, 1951c). 



2). Throughout this period of transforma- 

 tion, the fiagellum may remain attached to 

 the head and may undergo a very fine, inter- 

 mittent, vibratory motion, especially in the 

 region of the middle piece. The formation of 

 the definitive female pronucleus begins soon 

 after the second polar body has been com- 

 l)letely abstricted. The chromosomes re- 



maining within the ooplasm after extrusion 

 of the polar body are clumped together in 

 the form of a small, compact mass (Fig. 

 14.13). The first indication of transforma- 

 tion of this chromosomal mass into the fe- 

 male pronucleus is the appearance of several 

 minute nucleoli within a homogeneous nu- 

 cleoplasm. As the nucleoli increase in size 



