in the Foetus of Vertebrated Animals. 281 



dorsal roots of the aorta formed by the union of the brancliial 

 aVches on each side of the intestine. 



From the very cold state of the weather at the time I made 

 these observations, I found it very difficult to keep the chick 

 alive on the Held of the microscope, and to observe the circula- 

 tion of the blood going on, at the early stage of advancement 

 necessary in this investigation, and I was obliged to have re- 

 course to the plan of making transverse sections of the foetus in 

 the whole length of its body, in order to ascertain the structure 

 of its vessels, — a mode of observation by no means easy, but 

 one which affords most certain and satisfactory results. In this 

 manner, I have been enabled to confirm the general results 

 stated by M. Serres in regard to the double state of the aorta, 

 in the early stages of the development of the foetal bird. 



In the chick, at the 36th and at the 40th hours of incuba- 

 tion *, or a little before and immediately after the circulation of 

 the blood commences, I have seen two vessels rising from the 

 bulb of the heart, winding round the anterior portion of the in- 

 testine, and continuing to descend along the body of the foetus, 

 parallel to but separate from one another in their whole length. 

 These vessels are situated below the spinal marrow, and on 

 each side of the chorda dorsalis -f*, or part afterwards occupied 

 by the bodies of the vertebrae. The omphalo-mesenteric arte- 

 ries are given off' from these vessels considerably higher at this 

 than at a later period, and at first sight appear to be the only 

 branches continued from the aortic vessels ; but on minute exa- 

 mination, two other smaller vessels may be seen, situated be- 

 tween the omphalo-mesenterics, and descending some little way 

 below the place where these latter arteries pass off* into the vas- 

 cular area : towards the tail of the embryo, these two conti- 

 nuations of the aortic vessels seem to lose themselves in a large 

 vacant space left between the vascular layer of the germinal 

 membrane and the chorda dorsalis. 

 . Jn the chick at the 48th or 50th hours, or at the period when 



• In mentioning the hours of incubation, I state the period, not accord- 

 ing to the time occupied in the incubation of the individual foetuses em- 

 ployed, but according to the state of their advancement, and the general pe- 

 riods adopted by Baer, Prevost and Dumas, &c. 



t The Chorda dorsalis, so called by Baer, corresponds in its position to 

 the primitive streak of the cicatricula : it is a small dense cord, situated im- 

 mediately below the spinal marrow. 



JANUARY MARCH 1831. T 



