168 



THE ORGANS 



of endothelium which become hollowed out. In regard to the origin of the 

 later vessels in the extraembryonic area, and those within the embr^'o, there 

 are two views: (i) that they represent outgrowths from the original capillaries; 

 (2) that they arise in situ in the same manner as the earliest capillaries and 

 unite secondarily to form networks. The weight of evidence at present favors 

 the latter view. The entire vascular system is at first represented by a network 

 of capillaries. As development proceeds some of the channels enlarge to form 

 the arteries and veins. 



The heart first appears as an endothelial tube, the primitive endocardium, 

 at a very early age of embryonic life, in the chick embryo during the first day 

 of incubation. It apparently begins its rhythmic contraction before any 



Coelom 



Parietal mesoderm 



Ectoderm 



Visceral mesoderm Blood islands 



Fig. 98. — Section of Blastoderm of Chick of 42 Hours Incubation. Photograph. 

 The cells of the blood islands are differentiated into nucleated red blood cells (crythro- 

 blasts) and the endothelium of the vessels. 



contractile fibrils can be distinguished in its walls and before any connection 

 with blood-vessels has been established. The origin of the cardiac endothelium 

 is not definitely known. It is believed by some to be of entodermic origin, 

 by others of mesodermic, by still others to be partly derived from each of 

 these layers. Around this endothelial tube, but separated from it by a space, 

 there develops from mesoderm an entirely distinct muscular tube, the primitive 

 myocardium. These two tubes are at first united only in places by bands of 

 connective tissue. Later they unite so that the inner tube, the endocardium 

 becomes a lining for the outer tube, the myocardium. The union of the two 

 heart tubes occurs very early. The foregoing description applies to the chick 

 and to those mammals thus far studied. In the earliest human embryos 

 (2 to 3 mm.) the heart is already a single, slightly coiled tube connected at its 

 cephalic end with the ventral aorta and caudally with the omphalo-mesenteric 

 veins. The epicardium, as the visceral layer of the pericardium, has a sepa- 

 rate origin, being, constricted off from that portion of the mesoderm which 

 lines the primary body cavity. 



