42 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



lining of the intestine and the epithelial lining and epithelial cells of all of the 

 outgrowths of the intestine, which include the gill pouches and gills, the larynx, 

 windpipe, and lungs, the tonsils, the thyroid and thymus glands, the liver, the 

 gall bladder and bile duct, the pancreas, and the urinary bladder and adjacent 

 parts of the urogenital system. The student should note that only the epithelial 

 cells of these structures arise from the entoderm. 



H. THE FATE OF THE MESODERM AND THE FORMATION OF MESENCHYME 



i. Mesenchyme. In the further development of the mesoderm, mesen- 

 chyme plays an important role. Mesenchyme is not a germ layer but a particular 



ectoderm 



mesothelium 

 coelom of epimere 



kidney duct 



epimere 



mesenchyme of sclerotome 

 forming from epimere 



neural tube 



mesoderm 



notochord 



entoderm 



FIG. 9. Enlarged view of the epimere of a chick embryo of two days' incubation to show the 

 transformation of a portion of the epimere into mesenchyme. These mesenchyme cells constitute the 

 sclerotome from which the vertebral column arises. 



type of tissue. It is a primitive kind of connective tissue, consisting of branched 

 cells, whose branches are more or less united to form a network (see Fig. 9). 

 Nearly all of the mesenchyme comes from mesoderm; however, it may arise 

 from the other germ layers also. Hence tissues and structures which arise 

 from mesenchyme may owe their origin to more than one germ layer. Con- 

 sequently, to avoid inaccuracy it is usually merely stated that they arise from 

 mesenchyme, without specifying the particular germ layer or layers involved. 

 When a germ layer is about to produce mesenchyme its cells become loose, 

 separating from their fellows, lose their epithelial form, and taking on a branched 



