PHYLUM CHORDATA 



385 



through the common aperture, into one or other of the ovi- 

 ducts, where fertilisation (p. 393) occurs. As it passes into 

 the dilated portion of the oviduct, the oosperm (p. 60) of 

 Scyllium becomes surrounded by a horn-like egg-shell (Fig. 

 229), secreted by the shell-gland, and having the form of a 

 pillow-case produced at each of its 

 four corners into a long tendril-like 

 process. The eggs are laid among 

 seaweed, to which they become 

 attached by their tendrils. In some 

 other dogfishes {Acanthias, Mus- 

 telus) a mere vestige of the egg-shell 

 is formed, and the eggs undergo the 

 whole of their development in the 

 oviducts, the young being eventu- 

 ally born alive with the form and 

 proportions of the adult. 



The great size of the egg is due 

 to the immense quantity of yolk 

 which it contains ; its protoplasm 

 is almost entirely aggregated at one 

 pole in the form of a small disc. 

 When segmentation of the oosperm 

 takes place, it affects the protoplasm 

 alone, the inactive yolk taking no 

 part in the process. The disc of 

 protoplasm divides to form a little 

 heap of cells, the blastoderm, situ- 

 ated at one pole of the undivided sphere of yolk. The 

 blastoderm subsequently spreads out as a sheet of cells 

 over the yolk which it ultimately completely encloses. 

 While this extension of the blastoderm is taking place, its 

 middle part becomes raised up into a ridge-like thick- 



2C 



Fig. 229. —Dogfish, egg-case. 

 (After Dean.) 



