244 INTRODUCTION TO CYTOLOGY 



cessive life cycles in spite of such rearrangements is of the greatest impor- 

 tance in connection with problems of heredity. 



The Centrosome. — Shortly after the penetration of the spermatozoon 

 into the egg an aster develops in the cytoplasm near the base of the sperm 

 head, and at the focus of the aster a centriole appears (Figs. 145, 147). 

 The inclusion of a spermatid centriole near the nucleus during spermio- 

 genesis and the appearance of the aster and centriole in this general region 

 after fertilization led to the widely accepted view that the newly appearing 

 centriole is, in reality, that of the spermatid. Whatever its origin, it 

 soon divides into two which function in the first cleavage mitosis. These 

 facts had much to do with the formulation of a theory of fertilization 

 according to which the egg is not able to undergo division because of the 

 lack of any centrosome to initiate the process, while the spermatozoon 

 has a centrosome but not sufficient cytoplasm in which to act. Through 

 the union of the gametes all the organs necessary for division are brought 

 together and cleavage proceeds (Boveri, 1887, 1891). 



Various other theories regarding the origin of the cleavage centrioles 

 have been propounded, some workers identifying them with those of the 

 egg and others deriving them from both egg and sperm (see Wilson, 

 1925, p. 438). In practically all cases there are gaps in the known history 

 of these centrioles. It remains to be clearly proved that the cleavage 

 centrioles are continuous with those of either gamete, although in some 

 cases such proof is very closely approximated. On the other hand, it has 

 been shown that the formation of asters with centrioles can be induced in 

 the cytoplasm by treating the eggs with certain chemicals (see p. 154), 

 and that such structures, provided they originate near the nucleus, may 

 divide and function in the cleavage which follows,^* This suggests that 

 the spermatozoon may carry a substance which brings about centriole 

 formation by the cytoplasm in normal fertilization. However this may 

 be, the importance of the centrosome undoubtedly lies in its relation to 

 cleavage rather than to syngamy. 



Cytoplasm and Chondriosomes. — In animals, as in plants, the 

 evidence in general indicates that the cytoplasm of the male gamete is of 

 relatively little importance in syngamy. A considerable amount of pater- 

 nal cytoplasm is introduced in some cases {Ascaris), but in others none at 

 all can be seen to enter. This is particularly true of such a case as Nereis, 

 in which the middle piece and the tail remain outside the egg. These 

 facts are of interest in connection with the suggestion of Meves (1911) 

 that chondriosomes introduced by the sperm are responsible for the trans- 

 mission of certain paternal hereditary characteristics. The observed 

 behavior of these bodies during cleavage^^ does not support the theory. 



34 Tharaldsen (1926), Beams (1927). 



35 Van der Stricht (1902), Lams (1913), Meves (1914a). See Wilson (1925; p. 412). 



