FERTILIZATION 185 



chromosomes; (3) the centrosomes and asters 'derived in some 

 way, either directly or indirectly, from the spermatozoon. In 

 syngamy, therefore, the ovum supplies the great bulk of the 

 cytoplasmic basis of the zygote, together with one-half the 

 nuclear material, while the spermatozoon furnishes the other 

 half of the nuclear substance, and produces the centrosomes, 

 which here as elsewhere are to be regarded as the dynamic 

 centers for division. It should not be overlooked that a small 

 amount of cytoplasm, including certain mitochondrial structures, 

 does accompany the sperm, particularly when the tail piece 

 enters the egg; it is by no means impossible, though not at all 

 demonstrated, that this cytoplasm from the sperm may con- 

 tain substances of great importance in later development and 

 differentiation. In brief, however, it is true that in this union 

 of gametes the ovum is the material factor, the Spermatozoon 

 the dynamic, and each contributes equally to the nuclear or 

 controlling mechanism. 



But these structures are as yet distributed in different parts 

 of the cell. The association of the scattered elements into a 

 typical mitotic figure now follows and constitutes the final 

 step in fertilization and the formation of the new organism. 

 Maturation completed and the sperm nucleus dissolved, the 

 two germ nuclei commence to approach one another, the sperm 

 nucleus following the centrosomes and asters. The paths of 

 their approach are seldom directly toward one another, as 

 they are in some of the Nematodes, but are more or less 

 curved (Fig. 96), and seem in a way determined by some factors 

 other than mere mutual attraction, though this is doubtless the 

 essential factor in their movement (Wilson). 



The entrance path of the spermatozoon is frequently marked 

 by cytoplasmic modifications, often of a very pronounced 

 character, giving evidence of intense metabolic (katabolic) 

 activity; thus we might note the frequency of the accompanying 

 formation of pigment, which is usually regarded as a by-product 

 of protoplasmic decomposition. 



We have already seen that the path of the sperm nucleus 

 may be an important factor, either in determining or in making 



