132 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



MESEXCHYMATOU5 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 (Le., the re- 

 mains of the segmentation cavit}'^ 

 between the other body la} ers. 

 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. 138. The 

 mesench^Tne is characterized by 

 the fact that it never gives rise to 

 epithelial structures,^ and as a rule, 

 b}' 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. 



1 It is possible that the epitheliam (endothelium) lining the cavities of the viscular 

 system is of roesenchymatous origin, but ttie weight of evidence goes to show that some of 

 it at least is ot entj-dcrmic origin. 



Fig. 138. Section throngh the 

 head of an embryo Amblysloma, 

 showing the points, H and J/, 

 where the ectoderm is producing 

 the mesenchyme to form the hyoid 

 and mandibular arches. A, audi- 

 tory ganglion ; C, coelom of man- 

 dibular arch; C[., cnticnlar layer 

 of ectoderm ; MO, medulla oh>- 

 longata; ^\'/, nervous layer of 

 ectoderm; VII, facial nerve. 



