STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 69 



by a lighter area averaging 360 A in width (Odor and Renninger, 

 i960). Dense, probably basophilic, material was associated with the 

 outer surface of the membranes. The units could be spindle fibres 

 with thickened walls or tubular structures through which the fibres 

 pass. In the various phases of division, the spindle with its attached 

 chromosomes behaves as a solid body when extruded by rupture 

 of a living egg. The shape of the spindle varies greatly: it is short 

 and fat at metaphase and early anaphase, and long and narrow at 

 telophase. Sometimes the metaphase spindle comes clearly to points 

 at each pole, at other times it appears barrel-shaped. 



Centrosomes, centrioles and asters have been described in 

 mammalian eggs on several occasions : in the guinea-pig (Rubasch- 

 kin, 1905; Lams, 1913), bat (Van der Stricht, 1909), rat (Sobotta and 

 Burckhard, 19 10), cat (Van der Stricht, 191 1), dog (Van der Stricht 

 1923), rabbit (Amoroso and Parkes, 1948; Thibault, Dauzier and 

 Wintenberger, 1954; Dauzier and Thibault, 1956) and pig (Thibault, 

 i959)> but they are much less distinct than in non-mammalian eggs. 

 A suggestion of astral fibres can be seen in the rat egg shown in 

 Fig. 31. 



Components of the spermatozoon. In those animals in which the 

 sperm tail follows the head into the vitellus at fertilization, the 

 components of the tail, in addition to those parts of the head that 

 are not incorporated into the male pronucleus, dissociate and 

 evidently become part of the cytoplasmic equipment of the embryo. 

 The sperm tail has been reported to enter the vitellus in the eggs of 

 the guinea-pig (Hensen, 1876; Rubaschkin, 1905; Lams and 

 Doorme, 1908; Lams, 191 3), bat (Van der Stricht, 1902; Levi, 

 1915), mouse (Lams and Doorme, 1908; Gresson, 1940b, 1941), rat 

 (Sobotta and Burckhard, 1910; Van der Stricht, 1923; Kremer, 

 1924; Gilchrist and Pincus, 1932; Macdonald and Long, 1934; 

 Austin and Smiles, 1948; Blandau and Odor, 1952), dog (Van der 

 Stricht, 1923), rabbit (Nihoul, 1927; Pincus, 1930; Austin and 

 Bishop, 1957b), ferret (Mainland, 1930), pig (Pitkjanen, 1955; 

 Hancock, 1958; Thibault, 1959), golden hamster (Austin, 19560"; 

 Hamilton and Samuel, 1956; Ohnuki, 1959), field vole (Austin, 

 1957b), Chinese hamster, multimammate rat and Libyan jird (Austin 

 and Walton, i960). Nevertheless, entry of the tail cannot be 

 regarded as either universal or invariable in its occurrence : Rubasch- 

 kin, Sobotta and Burckhard, Nihoul and Pincus considered that it 

 did not always take place in the guinea-pig, rat and rabbit, Van der 



