116 KARL KKNST VON BAKU 



plates parts which contain in an undifferentiated .state the 

 skeletal and muscular systems, the connective tissues, and 

 the nerves belonging to these. In order to have a convenient 

 term for future use, I have named this layer the muscle- 

 layer "(p. 153)- 



The process of delarnination results then in the formation 

 of four layers, of which the upper two (composing the 

 " animal " or " serous " layer) will give origin to the animal 

 (neuromuscular) part of the body, the lower pair to the plastic 

 or vegetative organs. The uppermost layer will form the 

 external covering of the embryo, and also the amniotic 

 folds; from it there differentiates out at a very early stage 

 the rudiment of the central nervous system, forming a more 

 or less independent layer. Below the outermost layer lies 

 the layer from which are formed the muscular and skeletal 

 systems, and beneath this "muscle-layer" comes the 

 " vessel-layer," which gives origin to the main blood-vessels. 

 The innermost layer of the four will form the mucous 

 membrane of the alimentary canal and its dependencies; at 

 the present stage, however, it is, like the other layers, a 

 flat plate. 



From all these layers tubes are developed by the simple 

 bending round of their edges. The outermost layer becomes 

 the investing skin-tube of the embryo ; the layer for the 

 nervous system forms the tubular rudiment of the brain and 

 spinal cord ; the mucous layer curls round to form the 

 alimentary tube ; the muscle layer grows upwards and 

 downwards to form the fleshy and osseous tube of the body 

 wall ; even the vessel layer forms a tube investing the 

 alimentary canal, but a part of it goes to form the medial 

 " Gekrose," or mesenterial complex, which departs consider- 

 ably from the tubular form. 



When these tubes or " fundamental organs " are formed 

 the process of primary differentiation is compKir The 

 fundamental organs, however, have at no time actually the 

 form of tubes; they exist as tubes only ideally, for morpho- 

 logical and histological differentiation go on concurrently 

 with the process of primary differentiation. 



Through morphological differentiation the various parts 

 of the fundamental organs become specialised, through 



