41 6 MALL. [Vol. VIII. 



light is gradually being thrown upon the structure and archi- 

 tecture of the nervous system. 



From the purely anatomical standpoint we now ask ourselves 

 regarding every nerve, "from what cells does it arise.''" In 

 nearly all cases we find that the nerves have a mixed origin in 

 all more complex animals, and this should be the case, for only 

 with this mixture of nerves can the complexity of the animal 

 be brought about. In lower animals a nerve is often confined 

 wholly to one metamer while in the higher animals this is rarely 

 the case. No matter how great the mixture of nerves is, the 

 following generalizations apply to all nerves which have been 

 carefully studied. 



1. TJie primitive growing point of all vertebrate nerves is in 

 the layer of cells on the outennost side of the ectoderm and the 

 axis of division is parallel zuith the ectoderm. 



I do not wish to enter into many details regarding these 

 points in this paper, but add the following cases. It applies to 

 the whole central nervous system, to the sense organs of the 

 ectoderm and to the eye, ear and nose. In the latter, however, 

 the nerve cells sink into the thickened ectoderm of the olfactory 

 pit and then the planes of division can no longer be followed. 

 The spinal ganglia also caused me a great deal of trouble for the 

 relation of the plane of division to the ectoderm is lost as soon 

 as the mass of cells separates from it. Furthermore, the 

 ganglia of the tail of the animal do not arise directly from 

 the neural crest but by a system of sprouts from the more 

 anterior ganglia. In the latter case it is of course impossible 

 to follow the relation of the plane of division to the original 

 outside of the body. 



2. The direction of the transmission of an impulse is already 

 determined by the position of the cell in the ectoderm. 



The afferent, or receiving pole of the cell is on its free side, 

 i. e., the side which originally communicated with the exterior 

 of the body. The efferent, or giving pole is on the basal 

 side of the cell. The nerve fiber from either or both poles 

 may become very long and the nerves of the more complex 

 organs are made up of a chain of two or three cells, in which 

 the free side of one and the basal side of the other are more 



