118 KARL ERNST VON BAER 



Bichat's sense. The tissues and the germ-layers stand in 

 fairly close relation with one another, for while certain 

 tissues are formed chiefly but not exclusively in one layer, 

 others are formed only in one layer and never elsewhere. 

 For example, peripheral nerves are for the most part formed 

 in the muscle layer, though the bulk of the nervous tissue is 

 formed in the walls of the nerve tube ; similarly blood and 

 blood-vessels may arise from almost any layer, though their 

 chief seat of origin is the vessel-layer ; on the other hand, 

 bone is formed only in the muscle-layer (i., p. 155, ii., pp. 92-3). 



This relation of tissue to germ-layer was more fully 

 discussed and brought into greater prominence by Remak, 

 from the standpoint of the cell-theory, and it will occupy us 

 in a later chapter (Chap. XII.). 



The fourth Scholion elaborates the analysis of develop- 

 mental processes still further, and discusses in particular 

 the scheme of development which is shown by the Verte- 

 brata. The characteristic structure of the vertebrate body 

 is brought about by a "double symmetrical" rolling together 

 of the germ-layers, whereby two main tubes are formed, one 

 above and one below the axis of the body, which is the 

 chorda. The dorsal tube is formed by the two animal layers, 

 the ventral tube by all the layers combined (see Fig. 7). 



The process is indicated with sufficient clearness in the 

 diagram. It will be seen that the real foundation and 

 framework of the arrangement is the muscle-layer, with its 

 two tubes, one surrounding the central nervous system and 

 forming the "dorsal plates," the other surrounding the body 

 cavity and forming the "ventral plates." In the dorsal 

 plates, which early show metameric segmentation, the invest- 

 ing skeleton of the neural axis develops; in the ventral plates 

 are formed the ribs, the ventral arches of the vertebras, the 

 hyoid, the lower jaw and other skeletal structures. 



The alimentary or "mucous" tube and the part of the 

 vessel layer which invests it become so closely bound up with 

 one another as to form a single primitive organ the 

 alimentary canal. The muscles of the alimentary canal are 

 accordingly in all probability developed in the investing part 

 of the vessel layer. From the " Gekrose," or remaining part 

 of the vessel layer develop the Wolffian bodies (Urnicrui, 



