122 MAMMALIAN FERTILIZATION 



studied under a phase microscope. No obvious coalescence of 

 nucleoli was noted (Chang, unpublished), but a change of the 

 appearance of the cortex at the time of pronucleus formation was 

 observed (Chang, 1955b). 



Mark ( 1881 ) observed that the pronuclei of Limax eggs come 

 together but do not fuse to form a first cleavage nucleus and 

 stated that "the first cleavage nucleus does not have a morphologi- 

 cal existence." In the rabbit egg, Pincus (1939) stated that "the 

 stage intermediate between pronucleus juxtaposition and spindle 

 formation is rare and must therefore be gone through speedily." 

 In the rabbit eggs recovered at various times after mating the 

 membrane of two pronuclei is clearly seen especially with acetic 

 alcohol fixation but no nuclear membrane was observed before 

 the formation of the cleavage spindle (Chang, unpublished). In 

 Austin's beautiful photomicrographs (1951a), the close contact 

 of pronuclei and the metaphase and anaphase chromosomes are 

 well shown but a clear region without nuclear membrane inter- 

 preted as the prophase chromosomes is not convincing. Since 

 the maturation of pronuclei takes about 10 hours while meta- 

 phase, anaphase, telophase, and first cleavage take about 1 to 2 

 hours in the rabbit egg ( Chang, 1955b ) , it seems that the growth 

 or maturation of pronuclei corresponds to the prophase of an 

 ordinary cell division. A separate prophase stage after the con- 

 jugation of two pronuclei is still uncertain. Thus the point raised 

 by Mark still requires clarification in mammals. 



Since Boveri (1888) it has been known that in eggs of most 

 species of animals an important organ of cell division, the cen- 

 trosome, is lacking or inactive, and it is generally introduced by 

 the spermatozoon for the initiation of cell division. There is little 

 understanding of the centrosome in mammalian fertilization al- 

 though the acrosome of the spermatozoon in the rat egg has been 

 discussed (Blandau and Odor, 1950; Austin, 1951a). However, 

 if the centrosome contributed by the sperm is the organ for 

 cleavage, parthenogenetic division may be explained, as Tyler 

 ( 1941 ) has done for the invertebrates and lower vertebrates. 



