CHAPTER IV. 

 TELEOSTEI. 



THE majority of the Teleostei deposit their eggs before impregna- 

 tion, 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 Syngnathus 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. Ostegeniosus militaris, Arius falcarius, and Arius 

 fissus have this peculiar habit 



The ovum when laid is usually invested in the zona radiata only, 

 though a vitelliue 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 zoua 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 ovum 

 very much reduced in size. It seems in fact very probable that the 

 Teleostei are in reality derived from a type of Fish with a much 

 larger ovum. The occurrence of a meroblastic segmentation, in spite 

 of the ovum being usually smaller than that of Amphibia and Aci- 

 penser, etc., in which the segmentation is complete, as well as the solid 

 origin of many of the organs, receives its most plausible explanation 

 on this hypothesis. 



The proportion of the germinal disc to the whole ovum varies 

 considerably. In very small eggs, such as those of the Herring, the 

 disc may form as much as a fifth of the whole. 



