24 HISTOLOGY. 



the vessels from section to section they will be found to be branches. 

 The red blood corpuscles of the adult are thought to be descendants of 

 those which form the blood islands. They multiply in places to which 

 they have been carried by the circulating blood, for example in the liver 

 in later embryonic life, and in the red bone marrow of the adult. The 

 white corpuscles may be derived from the same parent form as the red, 

 or they may have several origins. The corpuscles pass out between the 

 cells of the vessel walls into the mesenchyma, where they wander about. 

 Whether some of them are formed by the transformation of mesenchymal 

 cells is still discussed. Their earliest origin like that of the vessel walls 

 is obscure. 



The vascular system in the chick embryo (Fig. 20) consists of the 

 network in the splanchnopleure just over the yolk, from which nutriment 

 is received by the blood. This is conveyed by the vitellinc veins, one on 



FIG. 23. WALL OP THE YOLK SAC (INTESTINE) FROM A CHICK OF THE SECOND DAY OF 

 INCUBATION. (Minot.) 



Mes., Splanchnic mesoderm; Ent., entoderm, four distinct cells of which are shown at c; V. V., blood 

 vessels containing a few young blood cells. 



either side, to the heart, a single median vessel under the pharynx made 

 by the junction of the veins (Fig. 20, B). The heart divides into two 

 aortae which pass around the anterior end of the pharynx to its dorsal side 

 and then extend through the bohy posteriorly, lying under the segments. 

 Their branches pass off laterally to the vitelline network, thus completing 

 the circulation. All future vessels in the body are branches of this simple 

 system. 



THE FUNDAMENTAL TISSUES. 



It has been said that there are two fundamental tissues, epithelium 

 and mesenchyma. Epithelium is a layer of cells covering an external or 

 an internal surface of the body, having one side free and the other resting 

 on underlying tissue. The epidermis, and the linings of the intestinal 

 tract, of the blood vessels, of the peritoneal cavity and of the joint cavities 



