XIII 



PHYLUM CHORDATA 



181 



large mass of yolk-spherules held together by means of a network 

 of protoplasmic threads, with, on one side, a disc of protoplasm— 

 the germinal disc. The process of maturation is similar to that 

 observable in holoblastic ova; one polar body is thrown off in 

 the ovary, the other apparently at impregnation. The ripe ovum 

 ruptures the wall of the enclosing follicle and so passes into the 

 abdominal cavity to enter one of the oviducts through the wide 

 abdominal opening. Impregnation takes place in the oviduct, and 

 the impregnated ovum in the oviparous forms becomes sur- 

 rounded by a layer of semi-fluid albumen and enclosed in a 

 shell of keratin secreted by the 

 shell-gland. The shell varies in 

 shape somewhat in the different 

 groups: most commonly, as in many 

 Dog-fishes (Fig. 826), it is four- 

 cornered, with twisted filamentous 

 appendages at the angles, by 

 means of which it becomes at- 

 tached to sea-weeds and the like. 

 In the Skates the filaments are 

 absent. In the Port Jackson 

 Sharks (Cestracion, Fig. 838) it is 

 an ovoid body, the wall of which 

 presents a broad, spiral flange. 

 The young Elasmobranch goes 

 through its development enclosed 

 in the shell, until it is fully formed, 

 when it escapes by rupturing the 

 latter. In the viviparous forms 

 the ovum undergoes its develop- 

 ment in the uterus, in which for 

 the most part it lies free — except 

 in some Mustelidse and Car- 

 charjidse, in which there is a close 

 connection between the yolk-sac 

 of the embryo and the wall of the 

 uterus, folds of the former interdigitating with folds of the latter, 

 and nourishment being thus conveyed from the vascular system of 

 the mother to that of the foetus by diffusion. In some of the 

 viviparous forms a distinct, though very dehcate shell, some- 

 times having rudiments of the filaments, is formed, and is 

 thrown off in the uterus. In the genera Bhinclahis and 

 Trygonwhina, which are both viviparou.=, each shell . encloses not 

 one egg, but three or four. Lsemargus is said to differ from 

 all the rest of the Elasmobranchii in having the ova fertilised 

 after they have been deposited, as well as in the small size of 

 the ova. 



Tig. SS8.— Egg-case of Cestracion 

 galeatus. (After Waite.) 



