MUSEUM OF COMPARATIVE ZOOLOGY. 179 



tween the two is due to the preponderance of the cell body in the dorsal 

 cells, which I have assumed to increase by virtue of its changed surround- 

 ings. A similar difference in the size of the cell body is found between 

 the fifth pair of ganglion cells, half projecting above the surface of the 

 brain, and the first pair, which is deeply embedded in its substance. 



One further point of interest suggests itself. It has already been 

 mentioned that the commissural cells vary in position, being in many 

 cases higher on the commissure than in others. If there be a tendency 

 toward a more dorsal position in this case as well, — which on the basis 

 of free space for development is more probable than the opposite move- 

 ment, — then there are two distinct ways in which originally ventral 

 elements may reach a dorsal position ; first by the independent dorsal 

 migration of superficially located ganglion cells, and secondly by a move- 

 ment of commissux-al cells dorsad along the commissure. The first method 

 is illustrated in the case of the dorsal cells, the second in the case of the 

 commissural cells (the third pair). In both instances the cause of the 

 migration may well be gain in nourishment and vigor as such cells 

 advance more and more into the free space above the brain mass. If 

 such a change in position involve a gain in vigor on the part of the 

 cells concerned, then the origin of a dorsal ganglion from a simple com- 

 missure by the dorsal migration of elements either independently or 

 along the commissure may be easily conceived, since around the ganglion 

 cells which have acquired this position other nervous elements will col- 

 lect with the increase in the amount of nervous matter accompanying 

 the development of the nervous system. In this way, then, lateral and 

 dorsal ganglia may arise. 



If the brain of Nectonema shows distantly how the higher develop- 

 ment may be reached, it shows still more clearly its immediate rela- 

 tion to the ventral nerve cord. The anterior ganglionic mass may be 

 viewed as a differentiation of the anterior portion of the cord. The 

 agreement between the portions of the brain and the three bundles of 

 the cord has already been emphasized. It remains to call attention to 

 the correspondence in the location of ganglion cells. As it was true 

 of the cord that ganglion cells were found on the borders of the three 

 fibrous tracts, so it is correct to say of the brain that the ganglion cells 

 are developed on the edges of the corresponding tracts. This will be 

 easily seen if, in a comparison of Figures 72-88 (Plate VT ), one pro- 

 ceeds from behind forward ; and it is still more evident when the ver- 

 tical bands of fibres are taken into account 3 the fibres are, however, not 

 represented in these figures. 



