m the Fvetus of VtriehmUd AnmaLs. 283 



class of animals in which the transformation occurs. In Mam- 

 malia, the aorta is formed by the permanence of the fourth 

 branchial arch and the aortic root of the left side ; in Birds by 

 that on the right; in the greater number of Reptiles by one on 

 each side; in the tailed Balrachia by three or four arches on 

 each side and by both roots ; in Osseous Fishes by four ; and 

 in the Sharks, Skates, &c. by all the five pairs of branchial ves- 

 sels and the two roots which are to be found in the early st^€^ 

 of development in the foetus *. 



The discovery of the double state of the dorsal and abdomi- 

 nal aorta in the very young foetus, made by M. Serres, must, 

 however, be regarded as very interesting, not only as it points 

 out a very singular change, little attended to before it was inves- 

 tigated by this author, taking place in the median arteries, but 

 also as it seems to afford an explanation of some varieties in the 

 placeof junction of the roots of the aorta, and in the origin of 

 the cceliac, mesenteric and other arteries, which occur in several 

 tribes of reptiles. 



The observations of this author in regard to the union of .the 

 double arteriae Basilares and Callosa^, will be read with equal in- 

 terest, as well as several curious facts mentioned by him respect- 

 ing the union of the principal venous trunks, and the varieties 

 of distribution of the vessels in the umbilical cord in some 

 mammiferous animals. ' A. T. . 



Bee. 30. 1830. 

 • See Fi^. 1. 9. 11. 14. 15. 19. 20. 30. 35. 39. in the two last plates. 



ERRATA. 



Part 1. page 304, line 35, fw takes read take 



307, ... 9, ... fig. 17 fig. 16. 



320, ... 9, ... 20. dc 20. c. 



326, ... 17, afier 8 and 9 add (from Baer) 



2. ... 9d, ... 15, etefe (figs.2. &3») ^ • r 



3. ... 267, ... 6, /or branchial r^arf brachial 



13, ... prominent. ... permanent, -iii 



At the foot add the following note: r 



In the page above, the formation of the Ductus arteriosus in mammalia 

 has been described in the manner in which Meckel sujjposed it to take place ; 

 but since writing this description, I have made some observations on the 

 foetus of the sow and sheep, which induce me to believe, along with Biu-dach, 

 that the opinion of Meckel is incorrect, and that the single ductus arteriosus 

 is formed in these and other mammiferous animals in the same manner as one 

 of the two ducts which exist in birds and some reptiles, viz. by the perma- 

 nence of one of the branchial arches. The fifth branchial arch on the left side, 

 (marked W in fig. 39 of last Plate), is the one which appears to give rise to 

 this communicating vessel in mammalia. 



T 2 ' 



