CHAPTER IX. 



THE DEVELOPMENT OF THE CONNECTIVE TISSUES AND THE 



SKELETAL SYSTEM. 



All the connective or supporting tissues of the body, except neuroglia, 

 are derived from the mesoderm. This Goes not imply, however, that all the 

 mesoderm is transformed into connective tissues; for such structures as the 

 endothelium of the blood vessels and lymphatic vessels, probably blood itself, 

 the epithelium lining the serous cavities, smooth and striated muscle, and a part 

 of the epithelium of the urogenital system are derived from mesoderm. 



Primitive groove 



FIG. ioo. Transverse section of chick embryo of 27 hours' incubation. Photograph. 



The origin of the mesoderm itself has been discussed elsewhere (p. 93). 

 In this connection it is sufficient to recall that it is situated between the ectoderm 

 and entoderm and consists of several layers of closely packed cells (Fig. ioo). 

 The axial portion in the neck and body regions becomes differentiated into the 

 mesodermic somites. At the same time a cleft (the coelom) separates the more 

 peripheral portion into a parietal and a visceral layer (Figs. 101 and 103). In 

 the head region where, in the higher animals, there is little or no indication of 



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