98 SUPPLEMENT ON 



in the form of bunches, at others like cords, or like net work ; 

 the roes are then transparent globules, within which the yolk 

 is visible. In this state they are fecundated by the fluid or 

 milt of the males who pass over them, and at that period 

 show the greatest activity : it is at this period also that many 

 ascend rivers, some live in pairs, others become gregarious 

 and migratory. The germ appears earlier or later in the 

 fecundated roe, according to the temperature, and is 

 growth is in general slow : the young fry, before it is much 

 increased in bulk, usually opens a passage by means of its 

 tail, through the covering membrane. In osseous viviparous 

 species, such as siluri, anableps, and certain blennies, the roe 

 swells in the ovarium, sufficiently for the growth of the foetus 

 within, and there are species where this growth is con- 

 siderable. 



All the egg roes contain beside the foetus also a vitellus, 

 which communicates by a pedicle with the intestine of the 

 foetus, and an external membrane similar to that of the eggs of 

 birds. No amnios has yet been observed, for the internal 

 tunic of the membrane, involving both fostus and vitellus, can 

 scarcely be admitted to be such. What constitutes the prin- 

 cipal distinction between the roe of fish, the spawn of 

 batrachi, and the eggs of other animals, who, when hatched, 

 are necessitated to breathe by lungs, is, that they are totally 

 devoid of the allantoid and of umbilical vessels, which are at 

 no time discernible ; hence, there is no placenta, and yet the 

 vitellus of the foetus of sharks appeared to the Baron much 

 contracted, and adhering to the uterus nearly as firmly as a 

 placenta, the cord being much ramified with vascular appen- 

 dices like a kind of coarse hair. 



When the fish is once brought into external life he is hence- 

 forth left to his own powers of subsisting. The far greater 

 number perish, devoured by larger fish, aquatic birds, and 

 reptiles ; the survivors increase in size with greater or less 



