906 FISHES—CYPRINODONTIDA. 
nective tissue cells and minute capillary blood vessels, with cellular 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 accompany ing 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 
larger 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 
parent. 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 fol‘icle in which the 
eggs have grown and matured; there is no true placenta, but respiration is effected by 
a follicular mesh-work of blood vessels, and the interchange of oxygen and carbonic 
dioxide gases takes place through the intermediation at first of the fluid 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 be 
conveyed to the uterine cavity before a perfectly normal development can begin; there 
is a fully developed richly vascular placenta joined to the fetus, 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 «ffected 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 ef 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 
sac. 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 built up from the protoplasm supplied from the blcod system of the 
parent. Judging from the large s'ze of the young of some viviparous fishes, such as in 
Embiotoca, ié 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 Zygonectes a large opening cf 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 
embryo I do not know, 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 
around 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 
