ORGANS OF REPRODUCTION. 



159 



coalesce into a single body, or one in which the division 

 is effected internally only by a more or less complete sep- 

 tum. Fixed by a mesoarium, the ovaries occupy generally 

 a position outwards of the intestine or air-bladder; their 

 form varies as well as the thickness and firmness of their 

 covering, which frequently is an extremely thin trans- 

 parent membrane. The inner surface of the ovarian sac 

 is transversely or longitudinally plaited or covered with 

 fringes, on which the ova are developed, as in the open 

 ovaries. In the viviparous Teleosteans the embryons are 

 likewise developed within the ovary, notably in the Embio- 

 tocid(E, many BlenniidcE, and Cyprinodontidce, Sebastes vivi- 



Fig. 70. — Ditrema argenteum, with fully developed young, ready for expulsion 

 by the genital orifice, o; a, folds of the ovariau sac ; v, vent. 



parus, etc. Among the Cyprinodonts the end of the oviduct 

 is attached to the anterior anal rays, which are modified 

 into supports of its terminatioa In Bhodeus the oviduct 

 is periodically prolonged into a long oviferous tube, by 

 means of which the female deposits her ova into the shells 

 of living Bivalves. 



The ova of Teleosteous Fishes are extremely variable in 

 size, quite independently of the size of the parent species. 

 The ova of large and small individuals of the same species, 

 of course, do not differ in size ; but, on the whole larger 



