ANIMAL AND VEGETATIVE ORGAN-SYSTEMS. 1 93 



of movement) proceed especially from the outer primary 

 germ-layer, from the skin-layer. The vegetative organ- 

 systems, on the other hand (the instruments of nutrition and 

 reproduction), proceed principally from the inner primary 

 germ-layer, from the intestinal layer. This radical contrast 

 between the animal and the vegetative spheres of the body 

 is, it is true, by no means absolute either in man or in the 

 higher animals ; on the contrary, many separate parts of the 

 animal apparatus (e.g., the intestinal nerve, or sympathetic) 

 originated from cells which have proceeded from the ento- 

 derm ; and, on the other hand, a large part of the vegetative 

 apparatus (e.g., the mouth-cavity, and probably the greater 

 part of the urinary and sexual organs) is formed of cells 

 which are originally derived from the exoderm. Moreover, 

 in the bodies of all the more highly developed animals, the 

 most heterogeneous parts are so intermixed and blended 

 that it is often extremely difficult to assign its true source 

 to each one of the constituent parts. But, on the whole, we 

 may assume as a certain and important fact, that in Man, 

 and in all high animals, the greater part of the animal organs 

 must be referred to the skin-layer, or exoderm ; the greater 

 part of the vegetative organs to the intestinal layer, or 

 entoderm. For this reason, Baer called the former the 

 animal germ-layer, the latter, the vegetative germ-layer 

 (Cf. vol. i. pp. 53 and 1 96). Of course, in making this important 

 assumption, we pre-suppose the correctness of Baer's view, 

 according to which the skin-fibrous layer (the ".flesh 

 stratum " of Baer) must have originated (phylogenetically) 

 from the exoderm, and, on the other hand, the intestinal- 

 fibrous layer (Baer's "vascular layer") from the entoderm. 

 This influential view, which is yet much disputed, is, we 



