1917] The Nervous System of Thysanura 311 



The cells are from one to three layers thick. The fibers in 

 the middle line gradually form into a thick mass of com- 

 missural strands. There are also a number of diagonal fibers 

 shown in the plane of the cross section. Farther down a second 

 commissure makes its appearance as a narrow area crossed by 

 coarse fibers. This crosses the center of the ganglion. Farther 

 along a marked, much arched commissure occupies a short 

 distance. The second commissure mentioned is quite extensive 

 but not very thick. More dorsal arched fibers show farther 

 along, these cross to some degree and come from lateral ventral 

 cells, in part at least. In about the central part of the ganglion 

 the other commissures and arched fibers have about disappeared 

 and two small central masses of commissural fibers are evident 

 and two ventral bundles of longitudinal fibers and marked 

 crossings from the ventral to the dorsal side are seen. Farther 

 along the two median commissures give place to one median 

 arched commissure, while many branches are seen at various 

 angles. This arched band disappears and another one comes 

 in contributed to by marked masses of lateral cells. A few 

 dorsal cells send their fibers straight into the ganglion from 

 above. Farther down a more dorsal arched commissure 

 comes in. Later there are two ventral straight bands of fibers 

 and then a single median band reaching from side to side, then 

 very soon the ganglion divides into the two ventral connectives. 

 Cells on the ventral and lateral sides are seen at all levels. 

 A few dorsal cells are seen near the central regions of the 

 ganglion. 



In the second thoracic ganglion a similar condition was 

 noted, at least nine commissures were counted. 



ABDOMINAL GANGLIA. 



A general summary of the structure of abdominal ganglia 

 of Lepisma will give an idea of their complexity : 



1. Cells chiefly ventral are found in from one to two layers. 

 The lateral ventral groups have three cell layers. There are a 

 few mid-dorsal cells of various sizes. 



2. There are in each ganglion a large number of com- 

 missures, both straight and arched, ventral and dorsal. 



3. Fibers cross dorso-ventrally and caudally. 



4. Fibers run short distances to nearby cell groups. 



