14 • THE SOMITES, NEPHROTOMY 

 AND LATERAL MESODERM 



I he 



he mesoderm is not as definitive a layer of tissue as the ectoderm and 

 the endoderm. The cells are more apt to migrate and to form loose aggregates 

 of single cells called mesenchyme. This mesenchyme spreads around other 

 structures and forms supporting tissue, connective tissue, cartilage, bone, and 

 in some cases muscle. 



The mesenchyme in early stages shows a very labile organization. Trans- 

 planted organs may stimulate it to form cartilaginous capsules. The ear 

 vesicle transplanted posteriorly induces foreign mesenchyme to form a 

 capsule of cartilage. More spectacular is the behavior of the mesenchyme in 

 the trunk region: in the amphibian this may be stimulated to form a limb 

 by the implantation of celluloid particles. The general picture is that of each 

 organ molding the mesenchyme into the general form of the organ. Bone 

 and cartilage are both very susceptible to pressure and tension. 



Development of the somites 



Let us first outline the general form of the mesoderm in the chick embryo 

 and then trace the development of the main divisions of this primary meso- 

 derm. The mesoderm of the early chick embryo (Fig. 116) shows three main 

 masses of cells: the somites, the nephrotome, and the lateral mesoderm. We 



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