594 THE EVOLUTION OF MAN. 



heart, and of the veins, which cany the blood back to the 

 heart. 



At first, the heart lies within the intestinal wall itself,, 

 from which it has developed, as do the first main blood- 

 vessels proceeding from it. The heart itself is in reality 

 only a local extension of one of these main blood-vessels. 

 Soon, however, the heart separates from its place of origin, 

 and now lies freely in a cavity, called the heart-cavity 

 (Figs. 145, Mi, 146, hh~). This heart-cavity is merely the 

 anterior part of the body-cavity (cosloma), which, as a 

 horseshoe-shaped arch, connects the right and left divisions 

 of the coelom (Fig. 140). The wall of the heart-cavity is 

 therefore formed, like that of the remainder of the body- 

 cavity, partly by the intestinal-fibrous layer (Fig. 146, df), 

 and partly by the skin-fibrous layer (hp). While the heart 

 is separating from the anterior intestine, it remains for a 

 short time attached to the latter by a thin plate, a heart- 

 mesentery (Fig. 146, hg). It afterwards lies quite freely in 

 the heart-cavity, and is directly connected with the intestinal 

 wall only by the main blood-vessels which pass from it. 



The anterior extremity of this spindle-shaped heart- 

 formation, which soon assumes a curved, S-shaped form,, 

 divides into a right and a left branch. These two tubes- 

 are arched and curved upward, and represent the two first 

 aortse-arches. They mount up in the wall of the anterior 

 intestine, which, in a measure, they encircle, and they there 

 unite above at the upper wall of the intestinal head-cavity 

 in one large single main artery, which passes backward 

 immediately under the notochord, and which is called the 

 main aorta (aorta principalis, Fig. 147, a). The first pair 

 of aortse-arches passes up on the inner wall of the first pair 



