70 THE MAMMALIAN EGG 



Stricht (1923) maintained that it did not occur in the cat, and Austin 

 found that entry failed in about 45 per cent of field- vole eggs under- 

 going fertilization (Fig. 24) and in the great majority of Chinese 

 hamster eggs. 



The tail of the spermatozoon may separate from the head soon 

 after entry into the vitellus and while the nucleus is taking on the 

 form of a male pronucleus, or it may remain attached to the pro- 

 nucleus for part or all of the pronuclear life span. In murine rodents, 



separation appears to be the rule, 

 whereas in the bat (Van der Stricht, 

 1902) and guinea-pig (Lams, 191 3) 

 the tail generally retains its attach- 

 ment. In the rabbit, the attachment 

 certainly seems to persist on some 

 occasions (Fig. 60). 



The components of the tail that 

 have been identified in the vitellus 

 are the centriole, mitochondria, 

 Golgi elements and the axial fila- 

 ments. The mitochondria and Golgi 



Male pronucleus in rabbit egg with dements become detached during 

 sperm tail still attached, x 900. fertilization or shortly thereafter and 



mingle with the particulates in the 

 egg cytoplasm (Gresson, 1940b, 1941) (Fig. 61). The tail filaments 

 are more persistent; they tend to become spread out as the outer 

 layers of the tail are lost (Fig. 62), and in the rat can be seen in 8-cell 

 eggs and even in the late blastocyst (Odor and Blandau, 1949). Sperm 

 centrioles have been reported in the eggs of the bat (Van der Stricht, 

 1909), rat (Sobotta and Burckhard, 1910), guinea-pig (Lams, 191 3), 

 dog (Van der Stricht, 1923), rabbit (Amoroso and Parkes, 1947) and 

 pig (Hancock, 1961). 



Of the parts of the sperm head that are not involved in pronucleus 

 formation, only the perforatorium clearly persists and is readily 

 traced in the vitelline cytoplasm (Fig. 17). (This body was called the 

 acrosome when it was originally described in the rat spermatozoon 

 by Lenhossek, 1898, but the term used here is now the more gener- 

 ally accepted; 'acrosome' is best reserved for the extranuclear cap.) 

 The perforatorium is perhaps best seen in the rat egg where it takes 

 the form of a short curved bifurcated rod; it can generally be 

 discerned throughout the period of fertilization and sometimes in 



