112 Incubation of the Top- Minnow. (Gambusia). [February, 



when it leaves the body of its parent. Nature will not waste her 

 powers in an effort to make useless clothes for such of her chil- 

 dren as do not need them ; on the contrary, she is constantly 

 utilizing structures economically, and often so as to serve more 

 than one purpose. This is the apparent answer to the query with 

 which we started. 



The follicles or sacs containing the ova are built up internally 

 of flat, polygonal cells of pavement epithelium, and externally of 

 a network of multipolar, fibrous, connective tissue cells and mi- 

 nute capillary blood vessels, with cellular walls, which radiate in 

 all directions over the follicle from the point where the main arte- 

 rial vessel joins the follicle, and which, together with its accom- 

 panying veins and investment of fibrous tissue, constitutes the 

 stalk by which the follicle and its contained naked ovum is sus- 

 pended 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 Cu- 

 vierian 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 follicle 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 ; be conveyed to the uterine cavity before a per- 

 fectly normal development can begin ; there is a fully developed 

 richly vascular placenta joined to the foetus, 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 imme- 

 diate vascular connection between mother and embryo. In both 

 the respiration of the embryo is effected by the transpiration of 



