3QO THE EVOLUTION OF MAN. 



body. Here we may speak of the first circulation of blood 

 in the embryo, and of its central organ, the heart. The first 

 blood-vessels and the heart, as well as the first blood itself, 

 develop from the intestinal-fibrous layer. On this account 

 the latter was called the vascular layer by the earlier em- 

 bryologists. In a certain sense this name is quite correct ; 

 :>nly it must not be understood to imply that all the blood- 

 vessels of the body proceed from this layer, or that the 

 . whole of the vascular layer is applied only to the formation 

 of the blood-vessels. Neither is the case. The intestinal- 

 fibrous layer also forms, as we saw, the whole fibrous and 

 muscular wall of the intestinal tube, and also the mesentery. 

 We shall presently find that blood-vessels can form in- 

 dependently from other parts, especially in the various parts 

 proceeding from the skin-fibrous layer. 



The heart and blood-vessels, and the whole vascular 

 system, are by no means among the oldest parts of the 

 animal organism. Aristotle assumed that the heart of 

 the Chick was formed first ; and many later authors have 

 shared this view. This is, however, by no means the case. 

 On the contrary, the most important parts of the body, the 

 four secondary germ-layers, the intestinal tube, and the 

 notochord, are already formed before the first indication of 

 the blood-vessel system appears. This fact, as we shall after- 

 wards find, is in complete harmony with the Phylogeny of 

 the animal kingdom, Our older animal ancestors possessed 

 neither blood nor heart. 



We have already examined the first blood-vessels of the 

 mammalian embryo in transverse sections. They are, first, 

 the two primary arteries, or "primitive aortae," "which lie 

 hi the narrow longitudinal cleft between the primitive 



