CRANIAL GANGLIA IN AMPHIOXUS. I 23 



remarkable penetrating qualities and is a faithful fixing agent, 

 there is no shrinking or swelling and the fibers are well fixed. 

 The bundle appears as an area of rather coarse fibers which take 

 a deeper stain than the remaining fibers in the dorsal region of 

 the cord. The bundle extends throughout the whole length of 

 the nerve cord, at least from nerve II. well into the tail region 

 where it grows very small. The bundle is very noticeably larger 

 on the right side than on the left and on both sides the bundles 

 increase in size toward the head end, perhaps because the ma- 

 jority of the fibers are ascending. In the head region the bundle 

 of the right side is augmented considerably by fibers from each 

 of the nerves VI. -III. inclusive. These fibers run forward and 

 mostly leave the bundle within one segment, for the nerve is 

 small immediately behind each of the nerves mentioned. In 

 brief, the sensory roots form three tracts in the dorsal part of the 

 cord, a diffuse mesial tract of fine fibers, a diffuse lateral tract of 

 coarser fibers and a compact dorsal superficial tract of coarse 

 fibers. The first and second mingle more or less with one 

 another and with fibers of other kinds running ventral to them. 

 The ventral limit of the dorsal tracts taken as a whole is roughly 

 marked by the lateral group of giant fibers. 



The very fine fibers seem never to be connected with ganglion 

 cells within the cord, but some of them do have their cells in the 

 nerve trunk. They run for a comparatively long distance in the 

 cord without dividing. Since the visceral sensory fibers in ver- 

 tebrates are fine, the hypothesis presents itself that these are the 

 visceral fibers in Auip/iio.vits. With this their position near the 

 dorsal raphe is consistent. Evidently the visceral fibers do not 

 enter the well defined bundle of coarser fibers ; for, if they did, 

 that bundle should be very large on the left side in the head 

 region where all the visceral surface is supplied by nerves of the 

 left side. The bundle is small on the left and larger on the right. 



The coarser fibers comport themselves in a variety of ways on 

 entering the cord. (In speaking of coarse and fine fibers of the 

 dorsal roots one must compare the central processes of ganglion 

 cells with central processes and peripheral processes with per- 

 ipheral processes and in order to do this one must know where 

 the ganglion cell of a given fiber is located.) Speaking then of 



