in the Fcetus of Vertehrated Animals. 311 



The auricle remains thin and membranous, becomes much di- 

 lated, and is placed behind the ventricle. The ventricle is now 

 much widened at its middle, and, from the projection of the 

 apex, begins to assume somewhat of a pyramidal shape. The 

 bulb of the aorta, the diameter of which remains comparatively 

 small, is now thrown forward on the neck, and we can easily 

 distinguish the arterial branches coming off from it on each 

 side. For some time before the tadpole leaves the water, its 

 heart seems to resemble almost entirely that of the adult ani- 

 mal : after this period, the apex of the ventricle is still more 

 prolonged, and the strength of its parietes increased ; but no 

 material change is known to take place when the animal begins 

 to breathe air only. 



The Heart of the Foetal Salamander may be perceived about 

 the same relative period as that of the frog. About the 5th or 

 6th day (Figs. 9, 10, 11), when the heart is first perceptible, this 

 organ has the form of a semitransparent and slightly curved 

 tube, lying across the body of the foetus in a depression between 

 the head and body (Fig. 9, h). At first, the contractions of the 

 heart, though regular, are very slow, there being, in the ordinary 

 temperature of the season at which they are developed (April and 

 May) only 20 or 25 pulsations in a minute. The contraction 

 begins at the end of the tube which is on the left side, and ex- 

 tends slowly and regularly through the whole tube till it reaches 

 the right side. The blood is at this time of a milky colour, and 

 can be seen best when the foetus is placed on a dark ground. 

 As the development of the animal proceeds, and the gills begin 

 to sprout out from the sides of the neck, the foetus leaves the 

 egg and swims about in the water. The blood now becomes of 

 a pinkish, and soon after of a red hue. The heart becomes 

 more distinct, and its contractions more rapid. The tube of the 

 heart is now altered in its form ; the auricle and ventricle and 

 bulb of the aorta become distinct from one another, and on lay- 

 ing the animal on its back and observing it with a moderately 

 strong lens, the circulation of the blood may be followed in al- 

 most all its parts (Figs. 12, 13). It is unnecessary to describe 

 more at length the changes which the heart of this animal un- 

 dergoes before arriving at its perfect state, as these changes ap- 

 pear to resemble very closely those just described in the frog. 



