Chap. XIII.] GONADS OF MAMMALIA. 507 



The next important step in the differentiation of 

 the testes is the great change in their topographical 

 relations which is to be observed in the higher 



o 



Mammalia, a change which is of the more signifi- 

 cance, as it is one that is never suffered by the ova- 

 ries. In the Monotremata the testes depart but little 

 from their primitive position ; in the porpoises they 

 lie beneath the kidney ; in the hedgehog they never 

 leave the abdomen, though they descend as far as the 

 inguinal ring ; * in the horse they pass through the 

 ring, which, however, remains permanently open, and 

 they make their way into a sack which is formed by 

 the integument, and is known as the scrotum. In 

 man, finally^ as in some other forms, the testes are 

 permanently enclosed in a scrotum, although in indi- 

 vidual cases of arrested development we have the 

 atavistic arrangement of the testes being throughout 

 life abdominal in position. 



The ovaries, like the testes, become more compact 

 as the scale of vertebrate organisation is ascended. In 

 both sexes we may find that the gonad of one side 

 projects farther forwards and that of the other farther 

 back. 



In rare cases, and in individual or specific varia- 

 tions (as in the toad, cod, herring), rudimentary testes 

 are to be observed in females, and rudimentary ova- 

 ries in males. On the other hand, Serranus, among 

 fishes, would appear to be ordinarily hermaphrodite, 

 and in individual cases only to have the sexes 

 separate. 



The spermatozoa of Elasmobranchs have, as 

 a rule, larger heads than those of Teleostean fishes ; 

 in the tritoii, salamander, and some other Amphibia a 

 remarkable undulating membrane is developed on the 



* The inguinal ring is the inferior or cutaneous orifice of the 

 inguinal canal, or space in the abdominal wall by which the 

 spermatic cord passes to the testis. 



