454 THE DOGFISH chap. 



In Acanthias and Mustelus (p. 415) 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 

 segmentation of the oosperm takes place it affects the 

 protoplasm alone, the inactive yolk taking no part in the 

 process (compare Crayfish, p. 369). The polyplast stage con- 

 sequently consists of a little mass of cells, the blastoderm 



Fig. 1 19. — Section of the upper part of the oosperm of a Dogfish which has undergone 

 segmentation to form the blastoderm. The bla-Stoderm 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 ; helow the blastoderm is the unsegmented yolk containing 

 scattered nuclei («). (From Balfour.) 



(Fig. 119), 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 to gradually 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 

 fish. The head, trunk, and tail acquire distinctness, and 

 become more and more completely separated off from the 

 bulk of the egg, the latter taking the form of a yolk-sac 

 (Fig. 120, A, yk.s) attached by a narrow stalk to the 

 ventral surface of the embryo. 



