498 



PISCES— FISHES, 



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 

 " 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. Fer- 

 tilisation occurs in the upper 

 part of the oviduct. Some 

 observers have described the 

 occurrence of polyspermy. 



Fig. 213. — Elasmobranch develop- 

 ment. — After Balfour. ;' 



Uppermost figure shows blastoderm at 

 an early stage. ; Ep,, Epjblas^j'Sjf.c, 

 segmentation -cavity ; »,, yolk-nuclei. 



Middle figure shows the invagination 

 which forms the gut. x. , Blastopore ; 

 g-., archenterpn. VM*sbderm .dark. 



Lowest figure, a longitudinal section at 

 a later stage. £#;, Eplblast ; «.c, 

 neural canal ; ne.c, neurenteric canal; 

 g., gut; «., notochord. Mesoderm 

 dark. 



As the ovum descends further, it 

 is surrounded first by albuminous 

 material, and then by the four-cor- 

 nered " 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 ( 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 

 serve very effectively to entangle the egg among sea-weed. 



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 subjacent yolk (Fig. 213, n.). 

 It seems most probable that these are hypoblast elements 

 which assist in the preparation of the yolk for absorption, 

 and eventually degenerate in the empty external yolk-sac. 



