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FISHES — CYPRIJiTODONTID^. 



neotive tissue cells and minnte capillary blood vessels, -with cellnlar walls, which 

 radiate in all directions over the follicle from the point where the main arterial vessel 

 joins the follicle, and which, together with its accompanjing veins and investment of 

 fibrous tissue, constitutes the stalk by which the follicle and its contained naked ovum 

 is suspended to the main arterial trunk and vein. The capillary system ends in a 

 Jarger venous trunk, which also follows the course of the main median arterial trunk 

 back to the heart by way of the Cuvierian ducts. The very intricate mesh- work of fine 

 vessels which covers the follicle supplies the developing fish with fresh oxygen, and also 

 serves to carry off the carbonic dioxide in much the same way as the placenta or after- 

 birth performs a similar duty for the young mammal developing in the uterus of its 

 iparent. There is this great difference, however, between the fish and the mammal. In 

 the former there is no uterus ; the development takes place in the follicle in which the' 

 eggs have grown and matured ; there is no true placenta, but respiration is effected by 

 a follicnlar mesh-work of blood vessels, and the interchange of oxygen and carbonic 

 dioxide gases takes place through the intermediation at first of the flaid by which the 

 embryo is surrounded in its follicle, and later, when blood vessels and gills have 

 developed in the embryo, they too become accessories to aid in the oxygenation of its 

 blood. In the mammal there is a uterus ; the egg must leave its ovarian follicle and he 

 conveyed to the uterine cavity before a perfectly normal development can begin ; there 

 is a fully developed richly vascular placenta joined to the fffitus, the villi or vascular 

 loops of which are insinuated between those developed on the maternal surface of the 

 uterine cavity. In both fish and mammal, however, this general likeness remains ; that 

 there is no immediate vascular connection between mother and embryo. In both the 

 respiration of the embryo is effected by the transpiration of gases through the inter- 

 mediation of membrane and fluids, oxygen being constantly supplied and carbonic 

 dioxide carried off by means of a specialized portion of the blood system of the maternal 

 organism. 



" There is still another difference which distinguishes the developing fish from the 

 mammal which has not been noticed. The body of the former is built up by gradual 

 transformation or conversion 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 a reorganization of the stored protoplasm of the yelk 

 eac. In the mammal, on the other hand, the embryo receives nourishment through the 

 placental structures, though there is a yelk at an early stage ; the largest proportion 

 of the embryo is bnilt up from the protoplasm supplied from the blcod system of the 

 parent. Judging from the large size of the young of some viviparous fishes, such as in 

 Embiotoca, it is possible that there may be some exceptions to the rule indicated above. 



" Besides the very intricate netVork of capillary vessels which covers the follicles of 

 the ovary of Zygoneotes a large opening of a circular or oval form makes its appearance 

 in the wall of each one at or near tbe point of attachment of the vascular stalk by which 

 4hey are supported. This opening appears to increase in size as the young fish develops ; 

 whether it is present during the earliest stages of the intrafoUicular development of the 

 embryo I do not knojv, as I did not have an opportunity to sea those phases. A branch 

 from the main nutritive vessel frequently-lies near the margin of the opening, curving 

 ground it. Whether this opening serves the same purpose as the micropyle of ova pro- 

 vided 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 in which the embryo 



