GERM LAYERS. 



M.T. 



Epi. 



1.5. 



tissue is called mesenchyma (Fig. 22). It fills the intervals between the 

 layers already described and surrounds the notochord and medullary 

 tube. Mesenchymal cells, however, do not enter the coelom. In the 

 chick embryo of Fig. 20, A, the greatest accumulation of mesenchyma 

 would be found between the ectoderm covering the head and the medullary 

 tube. Both the cells and the intercellular substance of mesenchyma 

 undergo transformations; the latter may become a more or less solid 

 matrix. Thus mesenchyma produces cartilage and bone, tendon, fascia, 

 and the loose connective 

 tissue through which the 

 vessels and nerves extend, 

 together with smooth 

 muscle fibers and fat. 



In the splanchno- 

 pleure, between the meso- 

 dermal and entodermal 

 layers, a network of blood 

 vessels, lined with very 

 flat cells, appears early 

 in embryonic life (Fig. 

 23). Its first indication 

 is the formation of irregu- 

 lar dark patches of cells, 

 called blood islands, which 

 surround the embryo as 

 a mottled layer. The 

 islands consist of cells 

 which form the blood cor- 

 puscles, and perhaps also 

 the lining of the blood 

 vessels which surround 

 them. So distinct is this 

 vascular layer that it has 

 been called the angioblast, 

 and regarded as a separate germ layer. Usually it is considered to be de- 

 rived from the mesenchyma. After the angioblast has once been developed 

 it sends prolongations into the embryo to form the blood vessels. The latter 

 thereafter never arise from mesenchymal spaces, but always as sprouts 

 from the pre-existing vessels, growing through mesenchyma like roots 

 through the soil. In single sections the lining of the vessels may appear 

 inseparable from the cells around them, as in Fig. 22, but by following 



B.V. 



FIG. 22. SECTION FROM THE HEAD OP A RABBIT EMBRYO OP io 



DAYS, 4.4 MM.. TO SHOW MESENCHYMA. 

 Epi. and M. T., Ectodermal epithelium of the epidermis and 



medullary tube, respectively. N., nucleus; P., protoplasm; 



and I. S., intercellular substance of a mesenchymal cell. 



Two of these cells show mitotic figures. B. V., Blood vessel, 



lined by endothelium. One of the blood vessels contains 



an embryonic red blood corpuscle. 



