44 HISTOLOGY 



in the splanchnopleure has formed a complete circulatory system. By 

 a process of folding, portions of the net have been brought together 

 under the fore-gut, where the vessels from the two sides have fused and 

 formed a single median tube, the heart. The two large trunks, derived 

 from the network, which convey the blood from the yolk-sac to the heart 

 are known asjntelline veins^ The heart divides anteriorly into two 

 vessels (the aorta) which pass from the under side of the fore-gut to the 

 upper side, and then extend posteriorly. They finally connect by 

 branches with the network over the yolk, from which they have been 

 derived. Through this system, nutriment taken from the yolk is brought 

 to the heart by the vitelline veins, and distributed throughout the body 

 by the aortae. 



In mammals also, a complete system of vessels is established early 

 in development, and it is believed that all later vessels arise as branches 

 of this primary endothelial network. If this opinion is correct, none of 

 the later vessels are formed by the coalescence of mesenchymal spaces, 

 or by transformation of mesenchymal cells into endothelial cells, but 

 only as outgrowths of pre-existing endothelium. There is, however, 

 a very close connection between the endothelium and the surrounding 

 mesenchyma, as shown in Fig. 31. 



The histogenesis of the blood is likewise very difficult to follow. The 

 simplest interpretation is one which has not been disproven, namely, that 

 all forms of blood corpuscles are descendants of the cells found in the 

 blood islands of the yolk-sac. According to this hypothesis these cells 

 multiply in certain places to which they have been carried by the circulating 

 blood, for example in the liver in later embryonic life and in the bone 

 marrow of the adult; and they differentiate into the red and white corpus- 

 cles of various kinds. The difficulties which this hypothesis encounters 

 will be discussed in later chapters. 



THE FUNDAMENTAL TISSUES. 



From the foregoing outline of embryological development, it is clear 

 that all the organs of the body are derived from a relatively small number 

 of fundamental tissues. After the fertilized egg has segmented, it gives 

 rise to layers of cells, of which the ectoderm and entoderm are epithelial 

 from the beginning. The mesoderm very early divides into two tissues 

 epithelium, which lines the body cavity, and mesenchyma, which forms 

 the internal substance of the body wall and intestinal wall. Thus epithe- 

 lium and mesenchyma may be regarded as the primary tissues of the body. 

 The groups of blood corpuscles, which are probably derived from the 

 mesenchyma, and the endothelium which surrounds them, also arise 

 very early, and these may be set apart as vascular tissue. 



