454 M. Vogt on some Inhabitants of the Freshwater Muscles. 



although still bearing the vitellary sac concealed in the ab- 

 domen. 



The fishes' eggs are, without doubt, introduced by the respi- 

 ratory current of the freshwater muscles. But their early exit 

 from the egg, at a period when the embryos of other fishes still 

 remain in the egg, as well as their whole manner of existence, 

 seem to me to prove that the gills of the freshwater muscles are 

 the habitual place of incubation of these embryos. They are all 

 concealed there in the same fashion, with the head turned toward 

 the free edge of the branchial lobes ; they thus fill the elongated 

 cavities between the two plates of a branchial lobe, and it is only 

 necessary to cut the external membrane in order to set the em- 

 bryos at liberty. It is then curious to follow the movements of 

 the oldest. After making some turns in the vessel containing 

 them, they return toward the gill, and eagerly attempt to pene- 

 trate into it. I have often seen them re-enter the respiratory 

 canal, and conceal themselves again in a branchial cavity where 

 they then kept quiet. 



I have not been able with complete certainty to ascertain to 

 what species of fish these little ones belong. The oldest which I 

 have met with had not yet any generic character ; they all still 

 possessed the embryonic fin continuous around the posterior ex- 

 tremity of the body, and the ventral ones were altogether want- 

 ing. But as I know the eggs of nearly all the genera of fishes 

 inhabiting our soft waters, I have reason to beUeve that these 

 eggs are of the Cottits Gobia, Linn., a species common in our 

 small rivers. 



These embryos are remarkably distinguished from all those 

 which I have hitherto observed ; the vitellus is almost opake and 

 of a yellow colour, which, under the microscope, appears of a 

 deep brown. The vitellary sac has a very elongated form, and 

 the young fish is lodged in a very deep depression of this enor- 

 mous vitellary sac. The difficulty of observation which results 

 from this disposition is further increased by two lateral swellings 

 of the yellow mass, swelhngs which rise where the pectoral fins 

 have to come out. The swelhngs of the vitellus enter indeed 

 into the base itself of the pectoral fin in the more advanced em- 

 bryos, and thus conceal all the anterior part of the body. To 

 examine the heart and the branchial region of the embryo, the 

 vitellary sac must be emptied, which soon causes the circulation 

 to cease. 



Embryonic researches among the inferior animals are often 

 only rendered so difficult by the want of proper materials. I 

 shall be happy if I have contributed to remove some of these dif- 

 ficulties, and I am sure that analogous researches on sea mol- 

 lusks may lead to numerous discoveries of the same kind. 



