XIII 



PHYLUM CHORDATA 



167 



opening into the urino-genital sinus to form an ovoid sac the 

 vesicula seminalis. A large thin- walled spei^m-sac is sometimes 

 present, opening close to the aperture of the vas deferens. The 

 Mlillerian ducts are vestigial in the male. 



Impregnation is internal in all the Elasmobranchs with the ex- 

 ception of Lsemargus (the Greenland Shark), the claspers acting 

 as intromittent organs by whose agency the semen is transmitted 

 into the interior of the oviducts. 



In all the Elasmobranchs the ova are very large, consisting of a 

 large mass of yolk-spherules held together by means of a network 

 of protoplasmic threads, with, on 

 one side, a disc of protoplasm 

 the germinal disc. The process 

 of maturation is similar to that 

 observable in holoblastic ova ; one 

 polar globule is thrown off in the 

 ovary, the other apparently at 

 impregnation./ The ripe ovum 

 ruptures the wall of the enclosing 

 follicle and so passes into the ab- 

 dominal cavity to enter one of the 

 oviducts through the wide ab- 

 dominal opening. Impregnation 

 takes place in the oviduct, and 

 the impregnated ovum in the 

 oviparous forms becomes sur- 

 rounded by a layer of semi-fluid 

 albumen and enclosed in a chitiii- 

 ous shell secreted by the shell- 

 gland. The shell varies in shape 

 somewhat rtr the different groups : 

 most commonly, as in the Dog- 

 fishes (Fig. 780),it is four-cornered, 

 with twisted filamentous append- 

 ages at the angles, by means of 

 which it becomes attached to sea- 

 weeds and the like. In the Skates the filaments are absent. In 

 the Port Jackson Sharks (Cestracion) (Fig. 791) it is an ovoid body, 

 the wall of which presents a broad spiral flange. Enclosed in 

 the shell, the young Elasmobranch goes through its develop- 

 ment until it is fully formed, when it escapes by rupturing 

 the egg-shell. In the viviparous forms the ovum undergoes 

 its development in the uterus, in which for the most part it lies 

 free, except jp some Mustelidce, in which there is a close connection 

 between the yolk-sac of the foetus and the Avail of the uterus, folds 

 of the former interdigitating with folds of the latter, and nourish- 

 ment being thus conveyed from the vascular system of the mother 



FIG. 791. Egg-case of Cestracionr-* 

 galeatus. (After Waite.) 9 



