In the process of fusion of the upper as well as the lower 

 parts of the embryo, the layers separate and acquire the form 

 of tubes inserted into each other, which Baer called the 

 basic of fundamental organs. 



The mucous layer forms the internal tube of the abdominal 

 half of the embryo. This fundamental organ Baer called the 

 ir;L coder ma] tube. It begins material exchange with the 

 external environment, specifically with the organs of digestion 

 and respiration. The vascular layer in the abdominal half 

 covers the mucodermal tube and forms two tubes: one lies 

 above the intestine and the other surrounds the intestine. 

 From this duplicated vascular-dermal tube, the blood vessels 

 form. The lower muscular layer of the animal layer also 

 forms two tubes: the spinal one includes the neural tube, 

 and the abdominal includes the previously mentioned two 

 vascular-dermal tubes. By means of histological differentia- 

 tion, the muscular layer divides into the dermofibrous and 

 the specific muscular layer. The central nervous system and 

 skin also form tubes: the skin tube envelops both muscular 

 tubes. Both have a common origin in the separated upper 

 layer of the animal part of the germ. Part of this initially 

 single formation appears in the interior (the neural tube) 

 and part remains at the periphery. 



The fact that these tubular basic organs are located one 

 inside the other, but become separate as different parts 

 undergo different activities, Baer tried to explain by the 

 polar opposition of their vital properties. This idea of 

 opposition as a source of formation undoubtedly has a nature- 

 philosophie character, and Baer considered it an echo of the 

 effect of Schelling and Cken on him. Baer, being sufficiently 

 sceptical about Naturphilosophie, nonetheless tried to find 

 in such ideas a rational seed and to use them for the discussion 

 of the phenomena he sought to understand. 



Baer used his scheme of embryonic development from systems 

 of tube-shaped basic or fundamental organs to establish a 

 general plan for the organization of vertebrates. Still more 

 important for this plan of organization Baer considered the 

 axes and planes of symmetry. Referring to his schematic 

 drawings, Baer described the relation of the basic (fundamental) 

 tubes to the main axis of the body. 



347 



