416 



THE MAINTENANCE OF SPECIES 



^ first and. 

 ^ ,® SeconoC 



polar 

 toocCy 



the head, middle, and tail pieces. 

 The head is composed chiefly of 

 chromatin and is usually more or 

 less pointed. The middle piece con- 

 stitutes the general region imme- 

 diately posterior to the head and 

 contains cytoplasm, m,itochondria, 

 centrioles, and the axial filament, 

 while the tail piece appears to be 

 primarily a locomotor device. 



Ova, on the other hand, are always 

 non-motile and much larger than 

 the sperm, due primarily to the fact 

 that ova contain nutritive material, 

 or yolk, which is utilized after fertil- 

 ization. The amount of yolk present 

 in eggs of the various classes of ver- 

 tebrates differs widely. In all forms 

 in which the eggs develop outside 

 of the body, as, for example, the 

 fish, amphibians, reptiles, and birds, 

 there must be enough nutritive mate- 

 rial present in the form of yolk to 

 supply the embryo until it hatches 

 and can feed itself. 



Fertilization 



Fertilization consists of the union 

 of a sperm and an ovum. This fusion 

 may occur either outside of the body 

 of the female, as in the case of most 

 of the teleost fishes and other water- 

 inhabiting animals, or within the 

 oviduct of the female. Literally 

 millions of sperm are liberated, but 

 usually only a single sperm enters an 



Generalized diagram of fertiliza- 

 tion. (I) shows the formation of the 

 first polar body, the maturation 

 spindle of the second maturation 

 division (see p. 429), and the pene- 

 tration of the spermatozoan. The 

 second polar body is formed by the 

 second maturation division and the 

 egg nucleus starts towards the cen- 

 ter of the egg. The sperm nucleus, 

 or male pronucleus, starts towards 



the center (II) via the entrance path, but turns (III) toward the center on its 

 copulation path to meet the egg nucleus and be arranged on the equatorial plate 

 (IV) for the first cleavage division. Note that the centrosomes for this division 

 are supplied by the male pronucleus. (After McEwen.) 



