Reproduction 



233 



Fig. 197. Spermatozoa of ape {Inuas), mouse (Mus), parrot {Psittacus), frog 

 (Ftana), and dogfish (Squalus). (H) Head; (M) middle piece; (T) tail. In the upper 

 of the two views of the ape spermatozoon, the thin, flat head is seen edgewise. 

 The spermatozoon of the frog is about 0.1 mm. long. (After Retzius. Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, The Rlakiston Company.) 



vidual from which arise buds. These become sexual adults which are 

 hermaphrodite — that is, they produce both eggs and sperm. 



In all vertebrates except a few fishes, the individual is either male or 

 female — the dioecious condition. The eel-like hag, Myxine (a cyclo- 

 stome or "round-mouthed" eel), and several of the bony fishes (Tele- 

 ostei) are normally hermaphrodite (monoecious). Among vertebrates 

 which are normally dioecious, many abnormal cases have been re- 

 ported, especially in fishes and amphibians, in which germ-cells of both 

 sexes were found in one individual. 



Germinal Bodies. The spermatozoa are derived from cells in 

 the walls of delicate tubules which are the essential part of the testis 

 (Fig. 195). The ova come from primordial germ-cells contained within 

 the tissues of the usually solid ovary (Fig. 196). 



The "head" of the spermatozoon (Fig. 197) consists of compacted 

 nuclear material (chromatin) derived from the primordial germ-cell. 

 A locomotor "tail" is formed from the cytoplasm (extranuclear proto- 

 plasm) of the original cell. The "ripe" spermatozoon is essentially a 

 motile nucleus. 



The egg in the course of its differentiation acquires a greatly in- 

 creased body of cytoplasm within which is deposited more or less food 

 material, the yolk or deutoplasm. The egg may become invested by 

 membranes or envelopes, either protective (e.g., the vitelline or yolk 

 membrane; the hard calcareous shell of a bird's egg — see Fig. 200) 

 or nutritive (e.g., the albumen or "white" of a bird's egg). 



Eggs differ most remarkably as to the amount of contained yolk 

 and as to their outer coverings. The microscopic egg of a mammal and 



