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 does 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, possibly 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. 138. Transverse section of chick embryo of 27 hours' incubation. Photograph. 



The origin of the mesoderm itself has been discussed elsewhere (p. 85). 

 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. 138). 

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

 primitive segments. At the same time a cleft (the coelom) separates the more 

 peripheral portion into a parietal and a visceral layer (Figs. 139 and 141). In 

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



165' 



