308 Dr Allen Thomson on the Vascular System 



tion of the intestine and the sac of the yolk. About the same 

 time that the heart can be perceived, the vascular network of 

 the yolk-bag begins to be formed. This network differs, how- 

 ever, entirely in its relations from the corresponding part in birds 

 or mammalia, as the mesenteric arteries do not reach it, and 

 veins alone are distributed on its surface (Fig. 7, Ic Jc*). 



Having thus shortly pointed out the changes which take place 

 in the germinal membrane of the ovum in the earlier stages of 

 development, I shall next describe the mode in which the heart 

 and bloodvessels are formed and evolved. It will be necessary 

 to divide this description into two parts : the first comprehend- 

 ing an account of the formation and development of the heart ; 

 the second that of the larger bloodvessels, more especially those 

 connected with the respiratory organs of the foetus. 



I. The heart, at the period of its first formation, in all ver- 

 tebrated animals which have been examined, is very similar in 

 its structure and in the situation which it occupies. The Rudi- 

 mentary parts of this organ are situated on the lower side of the 

 oesophageal portion of the intestine. Its primitive form is that 

 of a tube more or less bent, but equally simple in all vertebrated 

 animals. In these animals, therefore, the adult heart, however 

 varied its form, must be produced by some change in this tube ; 

 and from the facts I am about to relate, it will appear that these 

 changes are more or less complicated, according as the animal in 

 which they occur is higher or lower in the scale, or according 

 as its structure is more or less nearly allied to that of man. 



In treating this subject, we shall find it advantageous to be- 

 gin with the simpler class of animals, and then to proceed with 

 the more complicated. 



1. Development of' the Heart in Osseous Fishes. 

 According to the observations of Rathke on the Blennius, 

 the heart of the foetus of this fish appears, at the commence- 

 ment of its formation, like a small reniform body of a granular 



• Since writing the above, I Iiave seen the Memoir of M. Prevost, on the 

 Development of the Mulus gobio, in which the observations of Rathke are 

 confirmed in most particulars. 



