THE NERVOUS SYSTEM OF AMPHIBIA 169 



mesoderm immediately caudad of the thirty-seventh myotome 

 of the oldest specimen. Figure 51, however, is in reality taken 

 from the level of the unsegmented mesoderm immediately caudad 

 of the thirty-second myotome. Grouped according to specimens, 

 figures 28, 32, 36, 40 and 44 represent the embryo of the non- 

 motile stage; figures 29, 33, 37, 41, 45, 48, and 51, the early- 

 flexure stage; figures 30, 34, 38, 42, 46, 49, 52, the coil-reaction 

 stage; figures 31, 35, 39, 43, 47, 50, 53 and 54, the early-swimming 

 stage. 



(1) At the level of the third myotome. In figure 28, which is 

 taken from the level of the third myotome of the non-motile 

 embryo two giant ganglion cells are found in the most lateral 

 portion of the cord midway from its dorsal to its ventral limits. 

 These cells are filled with yolk spherules (not represented in 

 the figures) as are all the cells of this group in this stage. A 

 granular pigmentation outlines the margins of the cells with 

 their ventrally projecting processes. In contrast with the sur- 

 rounding cells, the nuclear plasma of the giant ganglion cells 

 takes on a faint tinge of the color used for counterstain of the 

 cytoplasmic elements and the chromatin has a finer structure. 

 In the latter part of the period under consideration the nuclei 

 of motor and commissural cells take on these same character- 

 istics in a considerable degree, but in this early period of develop- 

 ment the giant ganglion cells stand out as clearly the most dif- 

 ferentiated cells of the spinal cord. The processes of the giant 

 ganglion cells at this level in the youngest embryo are directed 

 ventral for a short distance, but it is impossible to detect long 

 fibers here or the presence of longitudinal fibers of the system, 

 either in transverse or longitudinal sections. 



In the corresponding level of the early-flexure stage, represented 

 in figure 29, the giant ganglion cell appears in a little more dorsal 

 position. Although no fiber tract can be seen in relation with 

 these cells in cross sections here, longitudinal sections show 

 spindle-shaped processes projecting from the giant ganglion cells 

 cephalad and caudad, forming a continuous column without a 

 differentiated fiber tract. Such a fiber tract becomes perceptible. 



