MEDULLA OBLONGATA OF AMBLYSTOMA 347 



surface for a considerable distance on each side of the mid-dorsal 

 plane. This is an embryonic character which is not preserved 

 in the adult. The mid-ventral plane, on the other hand, is 

 occupied by a thin layer of white substance containing the fibers 

 of the strong ventral commissure. 



The boundary between the gray and the white layers is an 

 intermediate region poor in cells, but rich in dendritic processes 

 and axones, many of which run taftigentially to the gray layer. This 

 is termed by Van Gehuchten the marginal zone of the gray sub- 

 stance. Within this layer are found occasional large spindle- 

 shaped correlation neurones, from both ends of which massive 

 dendrites arise and extend for long distances within the same 

 layer, giving off branches which may spread through practically 

 all parts of the white layer. Similar tangential neurones are found 

 in the medulla oblongata (where the marginal zone of the gray 

 substance is also present) and these probably represent the most 

 highly specialized types of correlation cells. 



At the periphery of the stratum album is a dense neuropil 

 formed chiefly of fine ramifications of dendrites of correlation 

 neurones. This is the plexus perimedullaris of Cajal, Lavdowsky 

 and Sala and is termed the marginal zone of the white substance 

 by Van Gehuchten. This layer is also present, though very 

 unequally developed in different regions, throughout the brain, 

 and within it the most important synapses appear to be developed. 



The neurones which give rise to ventral root fibers occupy a 

 ventral position in the gray layer and send large dendrites dor- 

 salward within the marginal layer of the gray substance, from 

 which branches are given off which reach practically all parts of 

 the white substance of the same side. Three small neurones of 

 this type with relatively short dendrites are seen in figures 17 and 

 18 (col.v.). The remainder of the gray substance contains corre- 

 lation neurones of several diverse forms, some of which also send 

 dendrites into all parts of the white substance. Many large 

 neurones of the ventral parts of the gray substance send big den- 

 drites through the ventral commissure to arborize in the stratum 

 album of the opposite side. Whether any of these give rise to 

 ventral root fibers I have not determined, and Van Gehuchten's 



