36 COMPARATIVE ANATOMY 



external bud-like outgrowths each of which develops into a sexual and 

 hermaphrodite adult. 



In the very great majority of vertebrates the individual animal is 

 differentiated as either a male or a female — the dioecious condition. 

 Among the lower vertebrates, however, there are a few animals which are 

 normally monoecious or hermaphrodite. The eel-like hag, Myxine, 

 of the class Cyclostomata, has a much elongated median gonad whose 

 anterior region may become a functional ovary while the posterior region 

 remains sexually inactive, or the anterior part may be inactive while the 

 posterior region differentiates as a testis. Self-fertilization is thus 

 impossible. That both regions become functional, but at different times, 

 in the life of the individual hag is not certainly known. It has been 

 claimed that the young hag is functionally male, but later in life becomes 

 female. Among the bony fishes (Teleostei) various hermaphrodite 

 conditions have been described. Serranus, a genus of perch, includes 

 fishes which are normally monoecious and self-fertilizing. Chrysophrys 

 aurata exhibits "successive hermaphroditism," producing eggs and sperm 

 alternately. 



Among fishes which are normally dioecious, the hermaphrodite con- 

 dition may sometimes occur as an individual variation or abnormality. 

 Such cases have been reported for several fishes, including such common 

 ones as the cod, herring and mackerel. So far as known, no vertebrate 

 above the fishes is regularly and normally hermaphrodite, but many 

 abnormal cases have been reported, especially in amphibians, in which 

 germ cells of both sexes have been found in one individual. 



THE GERMINAL BODIES 



The production of a new individual vertebrate animal is the outcome 

 of the joint activity of a male reproductive body, the spermatozoon, and a 

 female reproductive body, the egg or ovum. 



The spermatozoa are derived from cells in the walls of the dehcate 

 tubules which are the essential part of the testis (Fig. 27). The ova 

 come from primordial germ cells contained within the tissues of the usually 

 solid ovary (Fig. 28). 



The spermatozoon and the ovum each results from the differentiation 

 of a single cell of the gonad. In many respects their differentiation pro- 

 ceeds in quite opposite directions. Nevertheless each retains the essential 

 features of organization of a single cell. 



In the course of the differentiation of the spermatozoon (Fig. 29) the 

 cytoplasm (extra-nuclear protoplasm) becomes greatly reduced in volume, 

 very little remaining beyond what is required for the formation of a 

 locomotor apparatus which consists usually of a long filamentous "tail." 

 The "head" of the sperm cell consists of the compacted chromatin 



