THE PHENOMENA OF LIFE. 13 



It is found in the farther development of the animal that from each 

 of these layers is produced a very definite part of its completed body. 

 For example, from the cells of the epiblast, are derived, among other 

 structures, the skin and the central nervous system; from the mesoblast 

 is derived the flesh or muscles of the body, and from the hypoblast, the 

 epithelium of the alimentary canal and some of the chief glands. 



From the epiblast are ultimately developed the superficial skin or 

 epidermis and its various appendages, also the central or cerebro-spinal 

 nerve centres, the sensorial epithelium of the organs of special sense (the 

 eye, the ear, the nose), and the epithelium of the mouth and salivary 

 glands. 



From the JiypoUast is developed the epithelium of the whole diges- 

 tive canal, together with that lining the ducts of all the glands which 

 open into it ; also the glandular parenchyma of the glands (e.g., liver 

 and pancreas) connected with it, and the "epithelium of the respiratory 

 tract. 



FIG. 9. Transverse section through embryo chick (26 hours), a, epiblast ; 6. roesoblast ; c, hy- 

 poblast ; d, central portion of mesoblast, which is here fused with epiblast ; e, primitive groove ; 

 /, dorsal ridge. (Klein.) 



From the mesoUast are derived all the tissues and organs of the body 

 intervening between these two, the whole group of the connective tissues, 

 the muscles and the cerebro-spinal and sympathetic nerves, with the 

 vascular and genito-uriuary systems, and all the digestive canal, with its 

 various appendages, with the exception of the lining epithelium above 

 mentioned. 



It is obvious that these tissues and organs exhibit in a varying degree 

 the primary properties of protoplasm. The muscles, for example, de- 

 rived from certain cells of the mesoblast are highly contractile and re- 

 spond to stimuli readily, but they have little to do with digestion except 

 indirectly, and again, the cells of the liver, although doubtless contrac- 

 tile to a certain extent, yet have secretion and digestion for their chief 

 functions. 



Thus we see development in two directions going on side by side. It 

 speedily becomes necessary for the organism to depute to different 

 groups of cells, or their equivalents (i.e., to the tissues or organs to 

 which they give rise), special functions, so that the various functions 



