DEVELOPMENT OF SKATE 607 



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

 skates they resemble 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 ovi ducal 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 formative 

 protoplasm concentrated 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 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 pmrse " 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 seaweed, and moor the egg. There or in the mud, 

 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 secmrely 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 

 serve 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 

 blastoderm, or segmented area, some nuclei (so-called 

 *' merocytes ") are formed in the outer part of the sub- 

 jacent yolk (Fig. 343, n.). It seems most probable that 

 these are endodermal elements which^ assist in the prepara- 

 tion of the yolk for absorption, and eventually degenerate 

 in the empty external yolk-sac. 



At the close of segmentation the blastoderm is a lens- 

 shaped disc with two strata of cells. It is thicker at one 

 end — where the embryo begins to be formed. Towards 

 the other end, between the blastoderm and the yolk, lies a 

 segmentation cavity (Fig. 343, sg.c). 



