310 Dr Allen Thomson on the Vascular System 



preparatory to its entering the abdomen, or disappearing alto- 

 gether, the auricle advances forward so as to place itself be- 

 tween the ventricle and the vertebral column. In some fishes, 

 this change of position takes place only in a slight degree, so 

 that the auricle remains somewhat posterior to the ventricle ; 

 while in others it advances forward so as to lie immediately 

 above the ventricle, or even to come into contact with the bulb 

 of the aorta — a situation which it continues to occupy in the 

 adult fish. After this has taken place, and the valves at the 

 mouth of the aorta and auriculo-ventricular opening are formed, 

 the heart suffers little alteration, except in the increased thickness 

 of the ventricle and bulb of the aorta, and the dilatation of the 

 auricle and venous sinuses. 



2. Development of the Heart in Reptiles. 

 Though the dai'k colour of the embryo of the Frog and the 

 transparency of its blood, do not permit us to examine this fluid 

 circulating in the vessels, I have succeeded in separating the 

 heart from the embryo at a very early period. About the 5th 

 day, the heart of the tadpole, before it leaves the egg, consists 

 of a tube, which is so closely rolled up on itself as to give it 

 the appearance of a solid ball (Fig. 11). At this time there 

 appears to be little difference between the auricular and ven- 

 tricular parts of the tube. They are both composed of soft 

 dark-coloured granules loosely connected, so that considerable 

 care is necessary in unravelling them. The auricular part {v. 

 Fig. 11), is situated towards the left side of the foetus. The 

 ventricular part (*), occupies the greater curvature of the tube, 

 and stretches across the neck to the right side of the foetus, 

 where it is bent forwards, between the rudiments of the gills, so 

 as to form the bulb of the aorta {t), from which the principal ar- 

 terial vessels arise. About the 12th day, when the external 

 gills are nearly perfectly formed, the auricular becomes wider 

 than the ventricular part of the tube (Fig. 13). The ventricle is 

 also slightly dilated in its middle, and is separated by a con- 

 tracted part from the bulb of the aorta. Between the ^d and 3d 

 week, the colour of the blood is changed to red, and the altera- 

 tions in the form of the parts of the heart become more appa- 

 rent. About the third or fourth week, the primitive tubular 

 form of the heart can no longer be easily recognised (Fig. 16), 



