II. Organs. 10. RwproducMcm and Reproductive Organs. 33 



Compared with other cells the ovum is almost always large : in 

 many cases, p.g., the Mammalia, it is, however, microscopically small ; 

 but in others where there is a large quantity of deutoplasm, it attains 



Fig. 24. 

 a Titelline 

 protoplasm, 



Human OTiim. 

 membrane, h 

 c nucleus. — 



After KoUiker. 





Fig. 25. Diagram 

 of an ovum, with 

 many yolk-apherules. 

 d protoplasm, h, with 

 the nucleus, c, chiefly 

 accumulated at one 

 pole. — After Hert- 

 wig. 



ir-L 



a colossal size (Squalid^, Aves) . In this case it frequently happens 

 that most of the protoplasm, with the nucleus, accumulates at one 

 pole, whilst the chief mass of the egg consists of yolk held together 

 by the rest : or the protoplasm forms a thin layer over the whole 

 surface, whilst the inside consists principally of deutoplasm (Insecta). 

 The spermatozoon is, at first, a single small cell consisting of 

 protoplasm with a nucleus, and it may remain in this condition 

 (Fig. 26, 1) ; but 



3 4 



as a rule, when 

 mature, it is not 

 so simple : very 

 frequently it con- 

 sists of a thickened 

 part, the head, and 

 a long, thin, whip- 

 like tail, by the 

 lashing of which 

 it moves actively 

 along. The head 

 consists of the mo- 

 dified nucleus; the 

 protoplasm merely 

 forms the tail, 

 which may be re- 

 garded as a giant 

 flagellum. This 



Fig. 26. Spermatozoa of different animals, 1 Crustacean 

 (Thysanopus), 2 Crab, 3 Man, 4 Salamander (with a flattened- 

 out tail), 5 Beetle, h head (nucleus). — After different authors. 



kind of spermatozoon is met with in many animals belonging to 

 very different groups, but it may assume a variety of other forms. 



Genital Organs. — The formation of ova and spermatozoa is 

 usually confined to special regions of the body, and as a rule, but not 

 always, to definite organs. The organ in which the ova are formed is 



