xm PHYLUM CHORDATA 175 



which extends into the cloaca, and receives also the spermiducts : 

 it communicates with the general cavity of the cloaca by a median 

 opening situated on a papilla^-the urinogenital papilla. In the 

 female there is a median urinary sinus, into which the ureters open, 

 or the latter may open separately into the cloaca. 



Save in certain exceptional cases (e.g. Scy Ilium), there are two 

 ovaries, varying considerably in form, but always characterised 

 towards the breeding season by the great size of the follicles enclosing 

 the mature ova. The oviducts (Miillerian ducts) are quite separate 

 from the ovaries. The right and left oviducts come into close 

 relationship anteriorly, being united in the middle on the ventral 

 surface of the oesophagus, where each opens by a wide orifice into 

 the abdominal cavity, or both open by a single median aperture. 

 The following part of the oviduct is very narrow ; at one point it 

 exhibits a thickening, due to the presence in its walls of the follicles 

 of the shell-gland. Behind this is a dilated portion which acts as 

 a uterus, and this communicates with the cloaca through a wide 

 vagina. A considerable number of the Elasmobranchii are vivi- 

 parous, and in these the inner surface of the uterus is beset with 

 numerous vascular villi, while the shell-gland is small or vestigial. 



The testes are oval or elongate : the convoluted epididymis is 

 connected with the anterior end by efferent ducts, and from it 

 arises the vas deferens. The latter is dilated near its opening into 

 the urinogenital sinus to form an ovoid sac the vesicula seminalis. 

 A sperm-sac is sometimes present, opening close to the aperture of 

 the vas deferens. The Miillerian ducts are vestigial in the male. 



Impregnation is internal in all the Elasmobranchs with the 

 possible exception of LaBmargus (the Greenland Shark), the claspers 

 acting as intromittent organs by whose agency the semen is trans- 

 mitted 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 body 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 

 abdominal cavity to enter one of the oviducts through the wide 

 abdominal opening. Impregnation takes place in the oviduct, and 

 the impregnated ovum in the oviparous forms becomes surrounded 

 by a layer of semi-fluid albumen and enclosed in a shell of keratin 

 secreted by the shell-gland. The shell varies in shape somewhat 

 in the different groups : most commonly, as in many Dog-fishes 

 (Fig. 837), it is four-cornered, with twisted filamentous appendages 

 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. 849) it is an ovoid body, the wall 



