548 



PISCES FISHES. 



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 just behind the heart. About the middle of each 

 oviduct there is a large oviducal gland, which secretes the 

 horny "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 for- 

 mative protoplasm concen- 

 trated at one pole. 



The formation of polar 

 bodies (maturation) takes 

 place at an early stage. 

 Fertilisation occurs in the 

 upper part of the oviduct. 

 Some observers have de- 

 scribed the occurrence of 

 polyspermy. 



FIG. 290. Elasmobranch develop- 

 ment. After Balfour. 



Uppermost figure shows blastoderm at 



:2rJ&s%s?' . Asthe /r s desc H ends ,f rther> !t 



Middle figure shows the invagination is surrounded first by albuminous 



which forms the gut. jr., Blastopore ; material, and then by the four- 



*-., archenteron. Mesoderm dark. cornered "mermaid's purse" se- 



" 



ersage 



neural canal; *.., neurenteric canal ; gland. This purse is composed of 

 &., gut; ., notochord. Mesoderm keratin a common skeletal sub- 

 dark * stance which occurs for instance 



in hair and nails. Its corners 



are produced into long elastic tendrils, which may twine round 

 seaweed, and thus moor the egg. Rocked by the waves, the 

 embryo develops, and the young skate leaves the purse at one end. 

 Development is very slow, and takes perhaps the greater part of a year. 

 The egg-case of some sharks, e.g. the Port Jackson shark (Cestracion 

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

 the rocks ; that of a neighbour species ( C. galeatus] has reduced spirals 

 ending in a couple of tendrils, which may be 90 in. in length, and 

 serv'e very effectively to entangle the egg among seaweed. 



The segmentation is meroblastic, being confined to the 

 disc of formative protoplasm. From the edge of the 



