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On the Development of the Vascular System in the Foetus of 

 Veriehrated Animals. Part II. By Allen Thomson, M. D. 

 late President of the Royal Medical Society. Communicated 

 by the Author. ( Continued Jrom former volume^ p. 327. J 



xXAviNG in the former part of this essay considered the mode 

 of formation of the Heart in the different orders of vertebrated 

 animals, I shall now give some account of that of the other parts 

 of the vascular system'; viz. of the Bloodvessels of the body. 



There seem to be two modes principally in which bloodvessels 

 are developed ; the one,' by isolated points and vessels, has been 

 already alluded to, in- the account given of the commencement 

 of the circulation, as.it occurs on the vascular area of the yolk ; 

 the other, taking place after the commencement of the circula- 

 tion, by the prolongation of loops or folds from vessels already 

 formed, is most easily seen on the transparent parts of the 

 Batrachian reptiles. . . . '. , 



The sac of the yolk, or covering which: the, yolk receives from 

 the layers of the germinal membrane, is the; part ^ on which, in 

 all vertebrated animals, the blood- and vessels appear to origi- 

 nate, and it is the only part in which, in healthy aftimals, the 

 formation of bloodvessels has' been obiserved to take place inde- 

 pendently of thejieart or' gietneral circulation. . Durjng the de- 

 velopment of the vascular area (to the detail of ^ which jt is now 

 unnecessary to recur),Vno difference has as yet been observed 

 between the mode of ,theTormation of arteries and veins. The 

 blood appears to circulate sooner in the veins than in the arteries 

 of the area, but, in the early- stages' of development, these vessels 

 are to be distinguished from one, another only by their distri- 

 bution, and the direction of the currents of blood in them. 

 About the fourth or fifth day of incubation, the coats of the 

 arteries begin to appear thicker than those of the veins, and 

 very soon the external appearance of these vessels affords a 

 character sufficiently distinctive. As far as has been ascer- 

 tained, there does not appear to be any immediate connexion 

 between the formation of vessels in the area, and that of the 

 heart itself: these processes seem, for a time at first, to go on 

 simultaneously, but independently of one another ; and, in- 



