GERM LAYERS 



43 



and somatic layers of the mesoderm, but also from the somites. The 

 tissue which has been described as spreading from the somites around 

 the medullary tube, notochord and blood vessels, and into the deep 

 portion of the skin, is mesenchyma. It also surrounds the tubules derived 

 from the nephrotome. 



Under higher magnification, as in Fig. 31, it is seen that epithelium 

 is a layer of closely compacted cells, but that mesenchyma is a proto- 

 plasmic network, the meshes of which are filled with a fluid intercellular 

 substance. If this substance is abundant, the nuclei of the mesenchyma 

 are widely separated, as in the figure; but if it is scanty they are quite 

 close together. Mesenchyma gives rise to a great variety of tissues, 

 including involuntary muscle, adipose tissue, cartilage, and bone. Both 

 the cells and the intercellular substance may become variously modified. 

 The most widespread derivative of mesenchyma is connective tissue, 

 which invests the nerves, vessels, muscles and epithelial structures, bind- 

 ing them together in organs, and filling the interstices of the body. 



t Ent 



FIG. 32. WALL OF THE YOLK-SAC 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. 



The origin of the blood and blood vessels remains to be considered. 

 In very early stages the vessels appear as cellular strands, some of which 

 contain a lumen, situated between the mesoderm and entoderm. Asso- 

 ciated with these strands, but further out on the yolk-sac, there are 

 clusters or "islands" of blood cells, surrounded by a thin layer of flattened 

 cells known as endothelium. The entire system soon forms a network 

 of distinct vessels situated in the splanchnopleure (Figs. 29 and 32). 

 The formation of this primary vascular network in rabbit embryos has 

 been described by Bremer (Amer. Journ. of Anat., 1912, vol. 13, pp. 

 111-128). Generally the vessels and the corpuscles within them are 

 considered to be mesodermal, but some authorities have regarded them as 

 entodermal, and others have proposed to describe them as forming a 

 separate germ layer or "angioblast" (more appropriately angioderm). 



In the chick embryo shown in Figs. 27 and 28, the network of vessels 



