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Mr. A. H. Young and Dr. A. Robinson. 



" The Development and Morphology of the Vascular System 

 in Mammals. I. The Posterior End of the Aorta and the 

 Iliac Arteries." By Alfred H. Young, M.B., F.R.C.S., and 

 Arthur Robinson, M.D. Communicated by Sir William 

 Turner, F.R.S. Received January 21, — Read February 10, 

 1898. 



(Abstract.) 



Though numerous observations have been made on the develop- 

 ment of the systemic aorta and on the aortic arches, including 

 their modifications and transformations at the head end of the embryo, 

 but little attention has been given to the development and modifica- 

 tions of the primitive vessels and the aortic arches at the caudal end. 



The statement that the primitive aortas are prolonged backwards 

 from the dorsal region into the tail, and that, f using there, they form 

 a caudal aorta — the middle sacral artery — seems to be generallv 

 accepted by embryologists. Obviously, if this view is correct, the 

 iliac arteries are not formed from, and do not represent any part of, 

 the primitive aorta), and they are generally regarded as being seg- 

 mental in character. 



Previous observations on the comparative anatomy of the mam- 

 malian aorta and its terminal branches made by one of ourselves in 

 1891, seemed to show that the true posterior continuation of each 

 primitive dorsal aorta was to be found, not in the middle sacral 

 artery, but in the iliac and' Irypogastric trunks. It was impossible, 

 however, to arrive at any satisfactory conclusion on the question in 

 the absence of definite and precise information regarding the develop- 

 ment of the caudal end of the aorta and its branches. 



We therefore commenced a series of observations on the develop- 

 ment of the posterior parts of the main systemic vessels in the 

 rat, mouse, ferret, cat, and sheep. In the first three of these the 

 sections examined represent a fairly complete series of the early 

 stages of development. In the cat several embryos of two different 

 stages were referred to, whilst our sections of sheep embryos repre- 

 sented only one stage of development. 



In our description of these the embryos of each class are arranged 

 in groups according to the stage of development of their arterial 

 systems, and in each group the general stage of development is indi- 

 cated by a short statement of the condition of some of the main 

 organs. 



On developmental grounds alone the following conclusions, which 

 form a brief summary of the results of our observations, have been 

 arrived at : — 



1. The primitive aoitaa are paired trunks which pass at either 



