144 



SUB-GLASS ELASMOBKANCHII. 



are viviparous. The rest are oviparous. In the viviparous 

 forms a thin inconspicuous shell may be formed, but in many 

 there is no shell, and there is a placenta-like interdigitation 

 of folds of the yolk-sac, with corresponding depressions in the 

 uterine walls {Mustelus, Carcharias). 



The mucous membrane of the uterine portion of the oviduct is glandular 

 and in many viviparous forms is raised into vilU. In Pteroplataea micrura,* 

 some of these villi are especially long, and dip down into the wide spiracles. 

 Moreover, they possess glands which secrete into the pharynx of the 

 embryo an albuminous fluid, which passes into the intestine and serves 

 as nutriment. In other viviparous forms {e.g. Myliobatis nieuhofii, Try- 

 gon, etc.) the glandular mucosa of the uterias secretes an albuminous fluid, 

 which would appear to enter the alimentary canal of the embryo by the 

 spiracles, and to be absorbed in the intestine. It has been suggested 



that the long ex- 

 ternally project- 

 ing gill-filaments 

 of the embryo, 

 which are highly 

 vascular, may 

 assist in absorb- 

 i n g nutriment 

 from the uterine 

 fluids in the vivi- 

 !)arous forms. 



In Laemar- 

 gus the eggs 

 are said to be 

 laid without 

 a shell, and to 

 b e fertilised 



externally (Turner). In other cases sperm is introduced 

 into the female ducts by means of the claspers — though 

 the copulation has rarely been observed. The period of 

 gestation is of considerable length (seven to ten months, or 

 even more), and the young are born or hatched fully developed 

 and of considerable size. The cleavage is partial, closely re- 

 sembling that of Sauropsida, and there is no larval stage. The 

 development has been largely studied, and throws much light 

 on many morphological problems (e.g. the origin and relations 

 of the excretoiy organs, of nerves and muscles, cranial seg- 

 mentation, etc.). 



Fig. 82.— Embryo of Mustelus luens connected witli the wall Of the 

 oviduct by the yolk-sac (from Glaus, after J. Muller). 



* Akock, P.R.S., 49, 1891, p. 359, and 50, p. -202 



