280 INTRODUCTION TO CYTOLOGY 



(1889) that if the centrosome is a permanent cell organ the conjugation of 

 the gametes must involve not only a union of nuclei but also a union of 

 centrosomes (Wilson, p. 210). Fol (1891), in his work on echinoderm 

 eggs, thought that he observed just such a process, which he termed "The 

 Quadrille of the Centers." The egg centrosome and the centrosome 

 brought in by the spermatozoon were both supposed to divide, the prod- 

 ucts then fusing in pairs to form the two cleavage centrosomes. A simi- 

 lar thing was reported by certain other investigators, but none of the cases 

 stood the test of later work. Another theory now abandoned was that 

 advanced by Carnoy and Lebrun (1897), who also attempted to derive one 

 centrosome from each gamete. The cleavage centrosomes were thought 

 to arise de novo and separately, one inside each pronucleus, to migrate 

 thence into the cytoplasm. 



Much less confidence is now placed in the persistence of the spermatid 

 or egg centrosomes through the fertilization stages. Since the middle 

 piece, which is thought to contain the centrosome, does not enter the egg 

 at all in Nereis, it seems probable that the male nucleus in some way 

 induces the formation of asters and centrosomes by the egg cytoplasm. 

 Lillie found that even a portion of the sperm head will bring about this 

 effect. In Unio (Lillie 1897, 1898) and Crepidula (Conklin 1897) it seems 

 not unlikely that each pronucleus causes the formation of one cleavage 

 centrosome. In the sea urchin Wilson (1901) concluded that the cleav- 

 age centrosomes in all probability arise by the division of one which orig- 

 inates de novo at the nuclear membrane. In almost every case there are 

 gaps in the known history of the centrosome in fertilization, and it seems 

 very doubtful whether the cleavage centrosomes are continuous with 

 those of either gamete. This conclusion is supported by the fact that the 

 formation of asters with centrosomes in the egg cytoplasm can be arti- 

 ficially induced by treating the eggs with certain chemicals, such as weak 

 MgCl2. It is possible that the spermatozoon carries a substance which 

 brings about centrosome formation in a similar way. However this may 

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

 cleavage rather than to fertilization. 



Cytoplasm and Chondriosomes. In some cases (Nereis} no cytoplasm 

 can be shown to enter the egg with the spermatozoon, whereas in others 

 (A scans) a relatively large amount is brought in. Its great indefiniteness 

 in behavior makes it seem probable that it has no special significance in 

 the fertilization process. 



The importance of the chondriosomes in fertilization has been empha- 

 sized by Meves (1911, 1915), who finds that many of these bodies are 

 present in the large cytoplasmic mass accompanying the sperm nucleus 

 in A scaris, and that they mingle with the chondriosomes of the egg. 

 Meves (1908, 1915, 1918), together with other writers, accordingly thinks 

 that they are concerned in the transmission of certain hereditary char- 



