a 
MUSEUM OF COMPARATIVE ZOOLOGY. ig 
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 yary 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 commissural 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 VI), one pro- 
ceeds from behind forward ; and it is still more evident when the ver- 
tical bands of fibres are taken into account ; the fibres are, however, not 
represented in these figures. 
