174 G. E. COGHILL 



the cells are oriented with their long axis longitudinal in the 

 cord. 



(7) At the level of the thirty-third myotome. Figure 51, as 

 stated above, is drawn through the unsegmented mesoderm just 

 caudad of the thirty-second myotome in the embryo of the early- 

 flexure stage. The ventral half of the cord here is purely epi- 

 thelial in structure and the myotomes are situated ventrally 

 with reference to the more differentiated part of the cord. The 

 ganglion cells are closely related to the mesoderm as well as to 

 the skin. 



In the embryo of the coiled-reaction stage (fig. 52) the sensory 

 column consists of spindle-shaped cells occupying the dorsal half 

 of the cord, with their long axis longitudinally oriented. The 

 dorsal part of the cord here frays out into neural crest elements. 

 In the embryo of the early-swimming stage (fig. 53) the attenu- 

 ation of the spinal cord at the level of the thirty-third myotome 

 is noteworthy. The sensory column in this stage, also, occupies 

 the dorsal portion of the cord. The myotomes here extend well 

 dorsad, but in figure 54, which is taken from the level of the un- 

 segmented mesoderm just caudad of the thirty-seventh myotome, 

 the mesoderm is found far ventrad. The cord here is much 

 wider than it is five myotomes cephalad. In fact, a lateral dis- 

 tention of the spinal cord at the tip appears to be typical, and 

 in many cases, particularly in the younger embryos, there is 

 a perceptible tendency for the central canal to form ventricular 

 evaginations. This condition may simply be the result of a stress 

 upon the sides of the spinal cord exerted by the mesoderm in its 

 growth, for, as shown in later descriptions, there is a region of firm 

 attachment between the neural tube and the mesoderm in the 

 caudal region during the early periods of development. 



(5) Generalization upon the basis of these descriptions. The 

 foregoing description of the sensory column at arbitrarily se- 

 lected levels has been employed as a means of presenting briefly 

 the results of exhaustive study of serial sections in various planes, 

 and it is hoped, demonstrates sufficiently the basis for certain 

 generalizations concerning the nature and differentiation of this 



