DEVELOPMENT. 



527 



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 urogenital sinus, 

 whence the spermatozoa pass into the cloaca. Thence, in 

 copulation, theypass into the complex "claspers" of the male, 

 which are said to be 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 ovi- 

 ducts, which are united ante- 

 riorly just behind the heart. 

 About the middle of each 

 oviduct there is a large ovi- /^ 

 ducal 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, mostly 

 of yolk, with the formative 

 protoplasm concentrated at 

 one pole. 



The formation of polar 



bodies (maturation) takes place at an early stage. Fertilisa- 

 tion occurs in the upper part of the oviduct. Some ob- 

 servers have described the occurrence of polyspermy. 



As the ovum descends farther, it is surrounded first by albuminous 

 material, and then 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 egg-case of some sharks, e.g. the Port Jackson shark (Ccstracion 

 philippi) has elastic spiral fringes, and is found securely wedged among 



FIG. 255. Elasmobranch develop- 

 ment. After Balfour. 



Uppermost figure shows blastoderm at 

 an early stage. Ep. , Epiblast ; sg-.c., 

 segmentation cavity ; ., yolk-nuclei. 



Middle figure shows the invagination 

 which forms the gut. _r., Blastopore ; 

 g., archenteron. Mesoderm dark. 



Lowest figure, a longitudinal section at 

 a later stage. &]>, Epiblast ; n.c., 

 neural canal ; ne.c., neurenteric canal ; 

 g,, gut ; ., notochord. Mesoderm 

 dark. 



