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On tJie Development of the Vascular System in the Foetus of' 

 Vertehrated Animals*. By Allen Thomson, M.D. late 

 President of the Royal Medical Society. — Communicated by 

 the Author. 



Introduction. 



In studying the various forms which the organs circulating the 

 blood assume in the different orders of Vcrtebrated Animals, we 

 are apt to consider the varieties which present themselves in these 

 organs, from their simple state in Fishes, to their more compli- 

 cated structure in Mammalia, as wholly unconnected with one 

 another. The discoveries, however, that have lately been made 

 respecting the development of the circulating organs in the earlier 

 stages of foetal life, appear to illustrate the relation which these 

 varieties bear to one another, as well as to the other organs of 

 the animals in which they occur, more fully than could previ- 

 ously have been done by the most extended comparison of their 

 forms in adult animals. 



From the ease with which observations may be made on the 

 eggs of birds during incubation, many physiologists have availed 

 themselves of the opportunity thus afforded of studying the de- 

 velopment of the foetus and of its different organs. By the 

 earlier authors, accordingly, the production of the heart and 

 bloodvessels was studied in the chick alone ; and it was not till 

 a very late period, that the observation of physiologists was ex- 

 tended to the formation or evolution of these organs in the other 

 classes of vertebrated animals. 



From this circumstance it has necessarily arisen, that we are 

 at present much better acquainted with the mode of develop- 

 ment of the organs of the chick than of any mammiferous ani- 

 mal, and that many facts, often regarded as ascertained by ob- 

 •servation in the human embryo, have only been inferred by 

 analogical reasoning, from what is known to take place in the 

 eggs of birds. It might, at first sight, be supposed, that such a 

 mode of reasoning is inadmissible in prosecuting researches of this 

 nature ; but the more the structure of the ova of the different 

 classes of vertebrated animals has been studied, the more perfect 



• This Essay was the subject of the author's Inaugural Dissertation on 

 taking the degree of Doctor of Medicine in the University of Edinburgh, 

 July 1830. 



