1.-85. j PROCEEDINGS OF tlNlTEt) STATES NATIONAL MtJSEUM. 14? 



egg of Gambusia, we are iu a positiou to ask the question why such an 

 unique condition of affairs should exist in this case. The answer, it 

 would appear, we need not go far to seek. In the case of eggs which 

 ordinarily hatch in the water it is necessary that they shouUl be sup- 

 plied with a covering more or less firm and capable of protecting the 

 contained embryo, which, in the case of Gamhusia, is not needed, be- 

 cause the embryo is developed so as to be quite competent to take care 

 of itself as a very well organized little fish when it leaves the body of 

 its parent. Nature will not w^aste her powers in an effort to make use- 

 less clothes for such of her children as do not need them ; on the con- 

 trary, she is constantly utilizing structures economically, and often so 

 as to serve more than one purpose at one and the same time. 



The ovarian follicles of Gamhusia containing mature ova or foetuses 

 are built up internally of flat or squamous polygonal cells of pave- 

 ment epithelium, and externally of a net- work of multipolar, fibrous 

 connective tissue cells and minute capillar^' blood-vessels with cellular 

 walls, which radiate in all directions over the follicle. From the point 

 where the main arterial vessel enters it, this vessel, together with its 

 accompanying vein 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 minute structure of the fol- 

 licular membrane is shown in Fig. 14, Plate IX; the capillary system 

 of the follicles converges by way of the veinules which join the large 

 median ovarian vein which follows alongside the course of the ovarian 

 artery back to the heart. The very intricate mesh-work of fine vessels 

 which covers the follicle supplies the developing foitus with fresh oxy- 

 gen, and also serves to carry off" the carbonic dioxide iu 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 differ- 

 ence, however, between the foetal fish and the foetal 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 placenta, but 

 respiration is effected by a follicular mesh-work of blood-vessels, and 

 the interchange of oxygen and carbonic dioxide takes place through 

 the intermediation at first of the fluid by which the embryo is sur- 

 rounded in its follicle, and later, when blood-vessels and gills have de- 

 veloped in the embryo (see Fig. 17, Plate IX), they, too, become acces- 

 sories to aid in the oxygenation of the blood of the foetus. 



In the mammal, on the other hand, there is a uterus ; the egg must leave 

 its ovarian follicle ; be conveyed to the uterine cavity before a i^erfectly 

 normal development can begin ; there is a fully developed, richly vascular 

 placenta joined to the foetus, the villi or vascular loops of which are in- 

 sinuated between those developed on the maternal surface of the uterine 

 cavity. In both fish and mammal, however, the following general like- 

 ness remains, viz, that there is no immediate vascular connection be- 

 tween mother and embryo. In both the respiration of the embryo is 

 effected by the transpiration of gases through the intermediation of 



