THE VASCULAR SYSTEM. 39 1 



spinal cords, the side-plates, and the intestinal-glandular 

 layer (Figs. 92, ao, Fig. 95, 96, ao) ; and, second, the two 

 principal veins, or " cardinal veins," which appear somewhat 

 later, outside the former, above the primitive kidney ducts 

 (Fig. 96, vc, Fig. 141, VG). The primitive arteries seem to 

 arise by fission from the inner parts of the intestinal-fibrous 

 layer ; the primitive veins, on the contrary, from the outer 

 parts of the same layer. 



In just the same way, and in connection with these first 

 blood-vessels, the heart also arises from the intestinal- 

 fibrous layer, in the lower wall of the anterior intestine, 

 near the throat, at the place where the heart remains 

 throughout life in Fishes. Perhaps it will not seem very 

 poetic that the heart develops directly from the intestinal 

 wall. But the fact cannot be altered, and is also easily 

 comprehensible phylogeneticalry. The Vertebrates are, at 

 any rate, in this respect more aesthetic than the Mussels. 

 In these the heart remains permanently lying behind on the 

 wall of the rectum near the anus, so that the heart seems to 

 be penetrated by the rectum. 



Midway between the gill-arches of the two sides of the 

 head, and rather further back, at the throat of the embryo, a 

 wart-like thickening of the intestinal-fibrous layer develops 

 on the lower wall of the intestinal head cavity (Fig. 143,^). 

 This is the first rudiment of the heart. This swelling is 

 spindle-shaped, at first quite solid, and is formed entirely of 

 cells of the intestinal-fibrous layer. It afterwards, however, 

 curves in the form of an S (Fig. 144, c), and a little hollow 

 is formed in its centre, in consequence of the accumulation 

 of a small quantity of fluid between the central cells. 

 Some single cells of the wall separate from the rest and 



