DE VEL OPMENT. 493 



anteriorly by an epididymis. The tube of the epididymis is 

 continued into the vas deferens, which is dilated posteriorly 

 into a seminal vesicle and an adjacent sperm sac. Finally, 

 the two vasa deferentia open into the urinogenital sinus, 

 through which the spermatozoa pass into the cloaca. In 

 copulation, the complex "claspers" of the male are inserted 

 into the cloaca of the female. 



The female organs or ovaries lie on each side of the car- 

 dinal sinus, moored by a fold of peritoneum. In young 

 skates they are like the young testes, but in the adults they 

 are covered with large Graafian follicles, each containing an 

 ovum. The ripe ova burst into the body cavity, and enter 

 the single aperture of the oviducts, which are united an- 

 teriorly behind the heart. About the middle of each oviduct 

 there is a large oviducal gland, which secretes the " purse ; " 

 the elastic lower portions open into the cloaca. 



Development. 



The ripe ovum which bursts from the ovary is a large 

 sphere of yolk, with the formative protoplasm concentrated 

 at one pole. 



In Elasmobranchs formation of polar bodies (maturation) 

 takes place at an early stage. 



In the upper part of the oviduct the ovum is fertilised. 

 It is said by some that numerous spermatozoa often enter 

 the Elasmobranch ovum, although only one is actually 

 concerned in fertilisation. 



As the ovum descends further it is surrounded by 

 albuminous material, and by the four-cornered " mermaid's 

 purse " secreted by the walls of the oviducal gland. This 

 purse is composed of keratin a common skeletal substance 

 which occurs for instance in hair and nails. Its corners are 

 produced into long elastic tendrils, which may twine round 

 sea weed, and thus moor the egg. Rocked by the waves, 

 the embryo develops, and the young skate leaves the purse 

 at one end. 



The segmentation is meroblastic, being confined to the 

 disc of formative protoplasm. From the edge of the blasto- 

 derm, or segmented area, some nuclei (so-called "mero- 

 cytes ") are formed in the outer part of the subjacent yolk 

 (Fig. 164, .).- According to some, these yolk nuclei after- 



