in ike Fcetua of Vertebratcd Animals. 279 



The principal arteries which M. Serres describes as formed 

 and united in the manner alluded to are the Aorta, the Arteria 

 basilaris and Arteria callosa cerebri, and the Umbilical arteries 

 in the funis of the allantois ; and he adduces in support of his 

 conclusion observations on the structure of these arteries in the 

 foitus of birds and mammalia at an early stage of its advance- 

 ment, in cases of malformation, and in the different orders of 

 vertebrated animals in their adult state. 



In speaking of the formation of the aorta, M. Serres refers to 

 tlic observation made by the greater number of those who have 

 attended minutely to the development of the chick (more espe- 

 cially by Pander, Beitriige zur Entwickelungsgeschichte, &c. 

 § 13. pi, viii.), that, towards the 60th hour of incubation, the 

 aorta of the chick consists of two vessels quite separate from 

 one another, in the abdominal part of the vessel where it gives 

 off the arteries of the vascular area. 



At this period, the abdominal part of the embryo consists 

 simply of the rudimentary vertebral column inclosing the spinal 

 cord, of the lateral thickened parts of the serous layer of the 

 germinal membrane which form the plates of the abdomen, 

 and of the commencing intestinal folds on the lower surface, — 

 which parts are situated nearly in the same plane with the hori- 

 zontal part of the germinal membrane. About the middle of 

 this part of the embryo, the two arteries of the vascular area 

 are seen proceeding from it to the transparent and vascular 

 areas ; while the aortic branches, with which they communicate, 

 form two^ parallel vessels, situated one on each side of the rudi- 

 ments of the vertebrae, and extending from the part of the back 

 opftosite to the ventricle of the heart, where they are joined in- 

 to one trunk, to the end of the tail. 



Both Pander and M. Serres have given the name of Umbili- 

 cal to the arteries of the vascular area, a circumstance which has 

 in some measure tended to obscure their description of them. 

 Pander, indeed, forgetting that the proper umbilical arteries, dis- 

 tributed on the allantois, are produced from the pelvic portions 

 of the aorta, at a period considerably later than the vessels of the 

 area, supposes that the only difference between the structure 

 of the aorta in the foetus, and that in the adult animal, consists 

 in the greater height at which the division of this vessel into the 



