470 THE DOGFISH 



" Mermaid's purse " 1 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 (p. 431) a mere vestige of the 

 egg-shell is formed, and the eggs undergo the whole of 

 their development in the oviducts, the young being even- 

 tually 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 

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



FIG. 128. Section of the upper part of the oosperm of a Dogfish which has undergone 

 segmentation to form the blastoderm. The blastoderm 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. segmentation cavity ; below the blastoderm is the unsegmented yolk containing 

 scattered nuclei (). (From Balfour's Embryology.) 



segmentation of the oosperm takes place it affects the 

 protoplasmic part alone, the inactive yolk taking no part in 

 the process (compare Crayfish, p. 383). The polyplast stage 

 consequently consists of a little mass of cells, the blastoderm 

 (Fig. 128), at one pole of an undivided sphere of yolk. 

 The cells of the blastoderm become differentiated into the 

 three embryonic layers ectoderm, mesoderm, and endo- 

 derm. At the same time the blastoderm extends in a 

 peripheral direction so as gradually to cover the yolk, and 

 its middle part becomes raised up into a ridge-like thickening, 

 which is moulded, step by step, into the form of the embryo 



1 An egg is contained in the oviduct figured (Fig. 127 r>). 



