216 



PRINCIPLES OF ANIMAL BIOLOGY 



in which the nucleus is contained; a minute body, the mid-piece, behind 

 the head, from which in some animals a centrosome is known to develop 

 in the fertilized egg; and a whip-like tail used as a motile organ. Most 

 of the higher animals produce spermatozoa of this kind, but in a variety 

 of shapes as shown in Fig. 168 (A-E). In other animals the form may 

 be wholly different (F-J). 



Fertilization. — The dual function of the union of egg and sperm has 

 been pointed out in Chapter VIII, and some of the methods of insuring 

 that the two cells are brought together are described in Chapter IX. It 



I 



Fig. 168. — Different forms of spermatozoa. A, badger; B, sheldrake Tadorna; C, 

 sturgeon; D, flycatcher Miiscicapa; E, opossum; F, loJjster; G, crustacean Polyijhemus; 

 fl, crab Dromia; /, crab Porcellana; J, crustacean Ethusa; (A-D after Ballowdz; F after 

 Herrick; G after Zacharias; H-J after Grobben. From Wilson's The Cell in Development and 

 Inheritance, courtesy of Macmillan Co.) 



was shown in the latter chapter that the time of fertilization, relative to 

 the location of the egg, is variable; that is, the egg may be fertihzed while 

 still in the body of the female, or not until the egg is laid. In those 

 that are fertilized within the body of the female, fertihzation occurs in 

 the oviduct in the vertebrates, or as in some invertebrates while the egg 

 is still in the ovary. It is now possible also to relate the time of fertiliza- 

 tion to the stage of maturation which the egg or oocyte has reached when 

 the sperm enters. 



In Ascaris meyaloceyhala, parasitic in the horse, the sperm enters 

 the cytoplasm of the primary oocyte about the time of the formation of 



