.^80 Dr Alkn Thomson on the Vascular Si/stem 



(Jjjac arteries takes place ; but this, it is obvious, affords J>Oj, ex- 

 planation of the circumstance, that the arteries of the vascular 

 arv'a < > the yolk, (forming as their more recent and appropriate 

 pf^ipe.of Omphalo-mesenteric implies), continuations of the intes- 

 tinal arteries, are each of them given off by a separate branch 



, ^f^ the aorta. 



.0yi M. Serres has also observed, that, between the 40th and 50th 

 hours, or immediately after the circulation of the blood has com- 

 menced, the trunk of the aorta is double in its whole extent, 

 from the place at which its branches spring from the bulb of the 

 heart to the end of the tail ; and he affirms that it is by the 

 gradual union of these two vessels on the median line that the 

 single aorta of the adult is formed. 



Baer, the accuracy of whose researches on development we have 

 so often had occasion to admire, had also directed his attention to 

 the state of the aorta in the early stages of incubation, but appa- 

 rently without the same success. In his history of the develop- 

 ment of the chick (Repert. Gener. d'Anat. et de Physiol, tom. 8. 

 p. 72.), he informs us, that the two vessels into ^vhich the ven- 

 tricle of the heart propels its contents, towards the 40th hour, 



aiaving passed round the anterior part of the intestinal tube, 



'and proceeded some way along the inferior surface of the ver- 

 tebral column, probably reunite after having been separated for 



i^.certain space. He says, that this union cannot, however, be 

 Wasily shown at this period, because these vessels, on arriving 

 below the vertebral column, appear to lose their parietes, and 

 their contents are too transparent to enable us to trace their 

 course. He adds, that their union can, however, be easily de- 

 monstrated before the end of the second day. 



-^ These remarks of Baer, and the circumstance that M. Serres 

 makes no allusion in his description of the primitive double 

 state of the aorta, to the existence of the ten branchial subdi- 

 visions of this vessel discovered by Huschke, Rathke and Baer, 

 and described at p. 64. of this essay, and that he has given us 

 no information on the means he employed in making this very 

 difficult investigation, have made me think the repetition of the 

 observations of M. Serres necessary, in order not only to in- 



''<juire into their accuracy, but to endeavour to point out the re- 

 lations of the two aortic branches described by M. Serres, to the 



