•34 



ELASMOBRANCHII 



fluid, surrounds the egg when it is laid. In Laemargus, however, 

 fertilisation appears to be external, and the eggs are comparatively 

 small. This is probably a secondary modification. 



Many Selachians are viviparous. In these, the egg-case becomes 

 reduced to a thin membrane (Mustelus) or disappears altogether 

 (Torpedo), and the wall of the oviduct develops bunches of 

 secreting villi or cotyledons (Dumeril [124]). While the embryo 

 Trygonid or Myliobatid takes in the nutritive fluid through its 



■s/j. 



'IV. 



PteroplaUa micrura, HI. Schn. Portion of oviduct opened to show the embryo inside. 

 e.w, cut wall of oviduct; c, embryo; i.v, interna! wall with villi ; od, oviduct ; sp, spiracle of 

 embryo; v, long villi entering spiracle. , . l a— 



AjLOi^, ■■■•>. , 



mouth or spiracles, in Pteroplatea (Wood-Mason and Alcock [6, 

 498]) the maternal villi actually penetrate through the spiracles 

 into the alimentary canal of the embryo (Fig. 92). In some of the 

 Selachians with villate oviducts (Mustelus, Carchttrias) an intimate 

 connection is established between the wall of the oviduct and the 

 highly vascular yolk-sac. A placenta is thus formed in which 

 maternal villi fit closely into corresponding crypts in the embryonic 

 yolk-sac (J. Midler). 



A number of gill-lamellae become elongated into threads, 

 projecting to the exterior as transitory larval external gills, and 

 may serve as organs of absorption (Fig. 7G). 



