APPENDIX VIII. 313 



for the body. These currents come out from the heart by 

 the aorta and the carotid arteries, and return to the heart 

 by the anterior and posterior yolk veins. The two ante- 

 rior yolk veins disappear first, and after them the left pos- 

 terior vein. The hematogenous layer has completely 

 overrun the yolk, and there exist no capillary ramifications 

 except upon the latter. The pectoral fin,- which at first 

 was pendent, rises, and keeps up a continual motion. The 

 formation of cells is complete in the dorsal cord, and the 

 intercellular substance has almost entirely disappeared. 

 The liver begins to form, its communication with the in- 

 testine is very distinct, and capillary networks form in 

 its interior towards the end of this period. The posterior 

 yolk vein stretches along the lower front of the intestine, 

 and bends back in the neighborhood of the liver. The 

 odd fin which surrounds the body grows larger. The first 

 traces of the otoliths appear in the ears. The different 

 divisions of the heart are visible externally, and the rudi- 

 ments of the opercle become more and more distinct. 



From the thirty-first to the fortieth day the nose begins 

 to show very distinct outlines. The buccal cavity forms, 

 and on both sides can be seen the first rudiments of the 

 upper jaw, under the form of two prolongations. The 

 choroidal fissure closes, and the development of black pig- 

 ment in the eyes prevents any further study of them. The 

 branchial fissures appear one after the other, and each 

 of the branchial arches receives a vascular arch. At the 

 end of this period there are five arches, the first of which 

 is the hyoidal arch. The semicircular grooves begin to 

 form in the ears. The cells of the muscles are arranged 

 in threads. The cells of black pigment in the epidennoidal 

 layer of the back are seen to appear. The whole circula- 

 tion undergoes important modifications while penetrating 

 into the tail, where it gives birth to a cardinal vein. The 

 circulation of the head becomes symmetrical, the right 



