1 8 8 2 .] Incubation of the Top- Minnow ( Gambusia). 1 1 3 



gases through the intermediation of membranes and fluids, oxygen 

 being constantly supplied and carbonic dioxide carried off by 

 means of a specialized portion of the blood system of the mater- 

 nal organism. 



There is still another difference which distinguishes the develop- 

 ing fish from the mammal, which has not been noticed. The body 

 of the former is built up by a gradual transformation or conver- 

 sion of the substance of the yelk into the various structures 

 which make up its organization. In other words, the young fish 

 obtains no nutrition from its parent ; there is merely an incorpo- 

 ration of the stored protoplasm of the yelk sack. In the mammal, 

 on the other hand, the embryo receives nourishment through the 

 placental structures, the largest proportion of the embryo is built 

 up from the protoplasm supplied from the blood system of the 

 parent. Judging from the large size of the young of some vivipa- 

 rous fishes, such as in Embiotoca, it is possible that there may be 

 some exceptions to the rule indicated above. 



Besides the very intricate network of capillary vessels which 

 covers the follicles of the ovary of Gambusia, a large opening 

 of a circular or oval form makes its appearance in the wall of 

 each one at or near the point of attachment of the vascular stalk 

 by which they are supported. This opening appears to increase 

 in size as the young fish develops ; whether it is present during 

 the earliest stages of the intrafollicular development of the em- 

 bryo I do not know, as I did not have an opportunity to see 

 those phases. A branch from the main nutritive vessel frequently 

 lies near the margin of the opening, curving around it. Whether 

 this opening serves the same purpose as the micropyle of ova 

 provided with a membrane, would appear very probable, as it is 

 difficult to see in what other manner the milt, which is probably 

 introduced into the ovarian cavity by the male, could reach the 

 ovum through the wall of its follicle. The opening into the fol- 

 licle may be named the follicular foramen. Through it the cavity 

 m which the embryo lies is brought into direct communication 

 with the general ovarian space. 



We found ourselves unable to determine the species of the 

 form, the structure of which is described above ; none of those 

 described in Jordan's Manual appear to agree with our species. 

 It may be, as some of us have surmised, that the isolation of the 



sula of Virginia, for a great 



rjthof 



