132 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



MESENCHYMATOUS STRUCTURES. 



As was stated on a preceding page (p. 8), the mesenchyme 

 may arise from ectoderm, entoderm, or mesothelium, either by 

 the separation of isolated cells, as is usually the case, or by the 



immigration of large masses of 

 cells into the space (i.e., the re- 

 mains of the segmentation cavity) 

 between the other body layers. 

 This immigration in large masses 

 from the mesothelium is shown in 

 the formation of the sclerotomes 

 in Fig. 112, and from the ectoderm 

 into the region of the head to form 

 the gill cartilages in Fig. 1 38. The 

 mesenchyme is characterized by 

 the fact that it never gives rise to 

 epithelial structures, 1 and as a rule, 

 by the great development in it of 

 intercellular substance, as seen in 

 fibrous or areolar connective tissue, 

 cartilage, bone, blood, etc. Smooth 

 muscle tissue, however, is an ex- 

 ception in this respect. 



Besides the connective tissues 

 proper, which extend through all 

 parts of the body, forming a sup- 

 port and connection for tissues and 

 organs, the mesenchyme also gives 

 rise to most of the skeletal and circulatory structures. 



FIG. 138. Section through the 

 head of an embryo Amblystoma, 

 showing the points, H and Af, 

 where the ectoderm is producing 

 the mesenchyme to form the hyoid 

 and mandibular arches. A, audi- 

 tory ganglion ; C, ccelom of man- 

 dibular arch ; CL, cuticular layer 

 of ectoderm ; MO, medulla ob- 

 longata ; NL, nervous layer of 

 ectoderm ; VII, facial nrve. 



1 It is possible that the epithelium (endothelium) lining the cavities of the vascular 

 system is of mesenchymatous origin, but the weight of evidence goes to show that some of 

 it at least is of entodermic origin. 



