The Heart of the Sea-Turtle. 321 



and its pulsating venous extensions, tend to outlive the left 

 moiety and its corresponding venous parts. 



The death of the ventricle also takes place in a certain 

 segmentalorder, which is virtually the same in all cases, 

 and which is indicated by dotted lines and numbers (Plate, 

 fig. 5).* 



It will be seen from the above figure that the left side of 

 the ventricle dies before the right, and that the last seg- 

 ments to die are a superficial one, extending from the vessels 

 downwards, and another, involving the apex and a portion 

 extending obliquely upwards to the right of it ; speaking 

 generally, the cavum venosum is the last part of the ventri- 

 cle to die. From what has been said, it appears that as the 

 heart's vitality is being lowered, a more primitive condition 

 of things is reached, i.e., the heart comes to consist of the sinus, 

 the auricle, and a simplified ventricle ; or to put it otherwise, 

 the parts least dependent on the constant supply of nour- 

 ishment are those that are oldest in the development of the 

 heart, as those also of greatest independent rhythmic power ; 

 so that observation on the order in which any heart dies 

 may be a means of reading its developmental history. It 

 is more difficult to study this subject on the mammalian 

 heart, but Harvey long ago pointed out that the right 

 auricle was the last to die, and that the left ventricle was 

 the first, though he does not seem to have emphasized the 

 significance of this fact. 



"When in the animal scale among vertebrates a second 

 auricle is acquired, as it is first among the Dipnoi, it is small 

 and of comparatively much less functional importance than 

 the right. 



In the sea-turtle, not only is the right auricle endowed 

 with greater vitality than its fellow, but it is consjficuously 

 larger, the left, however, making a certain degree of advance, 

 as to size, on the condition existing in the Dipnoi. 



The ventricle in the sea-turtle is much more sensitive to a 

 stimulus than that of other Chelonians ; it also has much 

 less vitality, can bear deprivation of its regular nourishment 



* The plate referred to will appear in the continuation of this 

 paper in the next No. of this Journal, 



