XXIX DEVELOPMENT 397 



the oviducts, where fertilisation occurs. As it passes into, 

 the dilated portion of the oviduct the oosperm of Scyllium 

 becomes surrounded by a horn-like egg-shell or " mermaid's 

 purse " secreted by the shell-gland, and having the form of 

 a pillow-case produced at each of its four angles into a long, 

 tendril-like process. The eggs are laid among sea-weed, to 

 which they become attached by their tendrils. In Acanthias 

 and Mustelus a mere vestige of the egg-shell is formed, and 

 the eggs undergo the whole of their development in the 



Fig. io6. — Section of the upper part of the embryo of a Dogfish in 



the blastula stage. 



The blasiodcnn is formed of a single layer of ectoderm cells (white) 

 and of several rows of cells (shaded), which subsequently give rise to 

 endoderm and mesoderm : sg:, the blastocoele. 



Below the blastoderm is the unsegmented yolk containing scattered 

 nuclei («). 



(From Balfour.) 



oviducts, the young being eventually born alive with the 

 form and proportions of the adult. 



The great size of the egg is due to the immense quantity 

 of yolk it contains : its protoplasm is almost entirely aggre- 

 gated at one pole in the form of a small disc. When 

 segmentation of the oosperrr takes place it affects the 

 protoplasm alone, the inactive yolk, as in the Crayfish 

 (p. 344), taking no part in the process. The polyplast 

 stage consequently consists of a little heap of cells, called 

 the blastoderm (Fig. 106), at one pole of an undivided 



