CHAPTER IV. 



TELEOSTEI. 



THE majority of the Teleostei deposit their eggs before 

 impregnation, but some forms are viviparous, e.g. Blennius 

 viviparus. Not a few carry their eggs about ; but this operation 

 is with a few exceptions performed by the male. In Syngna- 

 thus the eggs are carried in a brood-pouch of the male situated 

 behind the anus. Amongst the Siluroids the male sometimes 

 carries the eggs in the throat above the gill clefts. Ostegenio- 

 sus militaris, Arius falcarius, and Arius fissus have this peculiar 

 habit. 



The ovum when laid is usually invested in the zona radiata 

 only, though a vitelline membrane is sometimes present in 

 addition, e.g. in the Herring. It is in most cases formed of a 

 central yolk mass, which may either be composed of a single 

 large vitelline sphere, or of distinct yolk spherules. The yolk 

 mass is usually invested by a granular protoplasmic layer, 

 which is especially thickened at one pole to form the germinal 

 disc. 



In the Herring's ovum the germinal disc is formed, as in 

 many Crustacea, at impregnation; the protoplasm which was 

 previously diffused through the egg becoming aggregated at the 

 germinal pole and round the periphery. 



Impregnation is external, and on its occurrence a contraction 

 of the vitellus takes place, so that a space is formed between 

 the vitellus and the zona radiata, which becomes filled with 

 fluid. 



The peculiarities in the development of the Teleostean ovum 

 can best be understood by regarding it as an Elasmobranch 



