DEVELOPMENT OF THE HEART. 379 



behind the first, and these are situated on the inner surface 

 of the second and third gill-arches. At length, a fourth and 

 a fifth pair appear behind the third (Fig. 304) ; but while 

 the latter are developing, the first two are again disappear- 

 ing by growing together (Fig. 305). The permanent main 

 arteries develop only from the three posterior arterial 

 arches (3, 4, 5, in Fig. 304), the lung-arteries from the last 

 (p ; Fig. 306). (Of. with this Fig. 302.) 



The human heart also (Fig. 314) develops exactly like that 

 of other Mammals. We have already considered the first prin- 

 ciples of its germ-history (vol. i. pp. 392-395, Figs. 143-147), 

 which essentially corresponds with its Phytogeny. 191 We saw 

 that the very first rudiment of the heart is a spindle-shaped 

 thickening of the intestinal-fibrous layer in the ventral wall 

 of the head-intestine (Fig. 143, df). This spindle-shaped 

 formation then becomes hollow, forms a simple pouch, and 

 separates from the place at which it originated, so that it 

 then lies freely in the cardiac cavity (Figs. 145, 146). This 

 pouch bends into the form of an S (Fig. 144, c), and, at the 

 same time, turns spirally on an imaginary axis, so that the 

 posterior part lies on the dorsal surface of the anterior 

 part. The combined yelk-veins open into its posterior 

 extremity ; from the anterior extremity proceed the arterial 

 arches (Fig. 150, voL i. p. 398). 



This first rudiment of the human heart, which encloses 

 a very simple cavity, corresponds to the heart of the As- 

 cidians, and must be regarded as a reproduction of the heart 

 of the Chordonia ; it now, however, separates into two, and 

 then three parts, thus exhibiting for a very brief period the 

 heart-structure of the Cyelostoma and of Fishes. The spiral 

 turn and curve of the heart increases, and, simultaneously, 



