254 Dr Allen Thomson on the Vascular System 



existence of gills in a rudimentary state, is indicated by the 

 structure of the pharynx and surrounding parts. In the early 

 periods of development, the distribution of the arteries especially, 

 which take their origin from the bulb of the aorta, bears a stri- 

 king resemblance to the primitive simple state in which these 

 vessels have been observed to exist in fishes and batrachia before 

 their gills are formed. In the embryo of the Lacerta agilis, 

 while the cavities of the heart are yet single, Baer has observed 

 that the aorta is divided into live pairs of vascular arches, which, 

 after winding round the sides of the pharynx, reunite with one 

 another above it to form the descending aorta. Baer and Rathke 

 have also observed, that while these subdivisions of the aorta— 

 which may very properly be called branchial arteries — exist, the 

 sides of the pharynx are penetrated by transverse fissures, on 

 each side of which runs one of the vessels rising from the bulb of 

 the aorta. The same appearances have been observed by Baer 

 in the foetus of the Coluber natrix, at a corresponding period 

 of its development. 



In these animals, however, the transverse plates of the neck, 

 and the branchial arteries passing along them, do not undergo 

 farther subdivision, or become more fully developed, like the gills 

 of fishes or batrachia ; on the contrary, the plates are gradually 

 incorporated with the parietes of the pharynx, the apertures are 

 entirely closed, and the vascular arches are converted, by the 

 obliteration of some parts and the enlargement of others, into 

 the arteries, which, in the adult animal, take their origin from 

 the right and left ventricles or cells of the heart. 



Unfortunately, the manner in which the farther development 

 of these parts takes place has not been observed, owing to the 

 difficulty of procuring the ova of the saurian and ophidian rep- 

 tiles at many different stages of foetal life. From the many 

 varieties in the distribution of the arteries rising from the heart 

 in these reptiles, it appears probable that this would prove a very 

 interesting subject for investigation ; and it may be remarked, 

 that no animals are better suited for observation, from the length 

 of time that the blood continues to circulate in the foetus after it 

 has been taken from the ovum. 



Two of the branchial arches appear to remain permanent, in 

 order to form the right and left roots of the aorta in the adult 



