66 FROM FISH TO PHILOSOPHER 



It is, however, not enough for the adult of a species to 

 solve any physiological problem unless it is also solved 

 for the embryo, and the urea-retention habitus requires 

 that the elasmobranch protect the egg and embryo 

 against the osmotic loss of water imtil such time as the 

 embryonic respiratory membranes and kidney develop 

 to the point where they are self-sufficient in respect to 

 the maintenance of water balance. To this end the elas- 

 mobranchs first covered the egg with a waterproof 

 (*cleidoic' = closed) case, secreted by a gland low down 

 in the oviduct. This case was usually supphed with 

 coiled tentacles to facilitate its attachment to well-venti- 

 lated seaweed. (Empty skate-egg cases are familiar 

 sights along the American shores of the North Atlantic. ) 

 A large, urea-rich yoDc sac and a sort of artificial lake, 

 equivalent to the amniotic fluid that bathes the embryos 

 of reptiles, birds, and mammals, is thus provided for the 

 young until they develop biochemical independence. 



All observers are agreed that oviparous reproduction 

 by means of the cleidoic egg is the primitive mode 

 among the elasmobranchs, though it persists in only a 

 few recent forms; in most famihes the egg is retained in 

 the oviduct of the mother until the embryo is mature, 

 the case being reduced to a thin diaphanous membrane 

 or replaced by a placenta of sorts, thus giving way to 

 *ovoviviparous' reproduction. Many curious specializa- 

 tions are found for transferring nourishment to the em- 

 bryo, ranging from a pseudoplacenta in certain sharks 

 (Carcharinoida) to vascular channels passing from the 

 mother s oviduct to the embryo's gills or intestinal tract 

 through the embryo's spiracles in certain of the rays 

 (Myliobatidae). 



But to enclose the egg in a waterproof covering, or 

 secondarily to develop ovo viviparous reproduction, re- 

 quires that the egg be fertihzed within the body of the 

 female, so that for the first time in vertebrate evolution 

 the organism had to resort to internal fertilization. In 

 order to effect internal fertilization, pelvic fins of some 



