48 



General Part. 



becomes nipped off from the rest of the layer, and takes up a position 

 between this and the epiblast (Fig. 38). 



Eig. 38. Diagrammatic figures in explanation of the formation of the mesoblast in 

 the Vertebrata (transverse sections) ; 1 youngest, 4 oldest stage, ei, en, m, epi-, hypo-, 

 meao-blast. — Orig. 



After the origin of the mesoblast, the organism consists of three 

 germ-layers, epiblast, mesoblast, and hypoblast. From these three 

 layers the different organs of the animal are formed; from the 

 mesoblast all the connective tissue, the skeleton* (in so far as it 

 is not a cuticular structure), the muscular tissue, the vascular system, 

 the excretory and genital organs; from the epiblast, the epidermis, 

 the nervous system, most of the sense organs ; from the hypoblast, 

 the epithelial lining of the whole or of the greater part of the 

 alimentary canal, and its glands. I he mesoblastic structures, there- 

 fore, constitute, at least in the higher animals, the main mass of the 

 body of the adult. 



Of the different systems of organs, only the development of 

 the nervous system and digestive tract is dealt with here, and quite 

 briefly. 



The central nervous system usually arises as a folding, 

 invagination or thickening of the epiblast, from which it separates later. 



Among the Yertebrata, for instance, 

 ■i S a long furrow-like fold is formed 



along the dorsal side of the animal, 

 and is the incipient central nervous 

 system (brain and spinal cord) ; it 

 loses its attachment later, and lies 

 beneath the skin, as a tube. In 

 some cases {e.g., Echinoderms and 

 Chsetopods) the primitive connec- 

 tion of the derivatives of the 

 epiblast (i.e., the nervous system 

 and the epidermis) is retained 

 throughout life, either over a large 

 area, or at least at certain points. 



Fig. 39. Diagrammatic transverse 

 section, showing the formation of the 

 nervous system and the notochord in 

 the Vertebrata. A young, B older stage. 

 ch Notochord, ek, en, m epi., hypo-, meso- 

 blast, n nervous system. — Orig. 



* The chorda dorsalis of the Vertebrata, however, develops from the hypoblast, as 

 a fold, which is later constricted off to form a cord, dorsal to the gut. 



