142 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [1885. 



am aware amongst existing tisb embryos. In tlie caudal, tbe pOvSterior 

 extremity of the aorta is prolonged backward, slightly bent upward, to 

 beyond the anterior ends of the rays where it divides into two branches, 

 a ventral and a dorsal one, each running nearly vertical to the axis of the 

 body along the base of the caudal. From their posterior sides these two 

 branches each give off about eleven or twelve secondary twigs which 

 run out along and between the rays through the interradial spaces to 

 supply the capillary mesh-work of the latter and the terminal lobes. 



The membranous fold which is continued forward dorsally and veu- 

 trally from the base of the caudal, and which is a remnant of the con- 

 tinuous median tin-fold of an earlier stage, is also very vascular, but 

 gets its blood supply from vessels arising from the aorta in front of the 

 caudal. The foregoing is the arrangement of the vascular system of 

 the tins in foetuses about 1 inch in length. On comparing this stage of 

 Micrometrus with AmpMstiehns 1^ inches long, it was observed that the 

 vascular apparatus above described had already begun to diminish in 

 importance, and in the adults nothing could be found which exactly 

 represented it. The conclusion, therefore, is that like the vessels which 

 render the skin of the foetal Embiotocoid so highly vascular, this system 

 atrophies for the most ])art after having temporarily subserved some 

 important function. 



The skin of the foetal Micrometrus was highly vascular, and it was 

 observed that special trunks passed outwards to the skin along the 

 sides, these vessels arising for the most part above the aorta from the 

 sides of the vessels which passed to the dorsal and the muscles of the 

 tail. The vascular network beneath the skin is remarkably well de- 

 fined and could obviously not be for the purpose of supplying the skin 

 alone with blood. 



The complex and unique vascular apparatus of the vertical tins of 

 Embiotocoid tish embryos it seems to me to be mainly for the purpose 

 of effecting respiration through the skin, and not for the puri)0se of 

 absorbing nutriment from the ovarian space as Dr. Blake, has supposed. 

 My reason for entertaining such an opinion is the fact that the mouth 

 of the fcetus is now open and that the throat is perforate, and that the 

 back part of the intestinal canal is widened and clothed internally with 

 extraordinarily long and densely crowded villi. In fact this part of the 

 intestine has been hypertrophied in the foetus, and while there was no 

 food found by me in the alimentary canal, I cannot but believe that 

 this jjart of the intestine is already functional and subserves all of the 

 purposes of the alimentary apparatus of the adult, and that the al- 

 buminous substances secreted in the ovarian cavity are swallowed by 

 the fcetus, and finally digested by the peculiarly organized hind gut 

 which 1 have described. There is, therefore, it seems to me, nothing left 

 to do for the produced vascular membranes of the fins and the skin 

 except to aid the gills in respiration, the interchange of gases between 

 the parent and the ftetus taking place through the fluids contained in the 

 ovarian cavitv, and in much the same manner as in Gamhusia. The 



