168 ^ G. E. COGHILL 



As introductory to the study of cross-section drawings of smaller 

 magnification it will be helpful to examine figure 27, which is 

 drawn at a magnification of 940 diameters. It is taken from the 

 level between fifth and sixth myotomes of an embryo of the 

 early-swimming stage. The unipolar condition of the giant 

 ganglion cell in the latero-dorsal region is clearly brought out 

 in relation to the sensory tract and the peripheral fiber. The 

 latter projects out into the myoseptum. In the latero-ventral 

 region motor neurones may be recognized, and, farther dorsad, 

 a group of commissural or associative neuroblasts. There is no 

 perceptible commissure at this level. In the latero-ventral angle 

 of the cord is seen the motor tract, which is differentiated from 

 the sensory tract by its more open or vesicular condition. This 

 is probably due to its being composed of larger fibers than those 

 of the sensory tract. The sensory tract is composed of a thin sheet 

 of fibers situated immediately against the external limiting mem- 

 brane of the cord and extending from the dorsal border of the 

 motor tract to the process of the giant ganglion cell. Reference to 

 figure 27a will show the dendritic nature of the peripheral process 

 of the giant ganglion cells, since the contents of the perikaryon, 

 the yolk spherules, are found distributed out into the fiber a con- 

 siderable distance beyond the limits of the cord. Many more 

 pronounced illustrations of this condition are found in my prep- 

 arations. 



Figures 28 to 54, now to be described, are taken from the 

 same specimens from which the projections of figures 56 to 59 

 were made, and should be studied in connection with these 

 projections. They should, also, be studied in groups according 

 to the following considerations: figures 28 to 31 are from the 

 level of the third myotome of the several specimens; figures 32 

 to 35, from the level of the eighth myotome; figures 36 to 39, 

 from the level of the thirteenth myotome; figures 40 to 43 from 

 the level of the eighteenth myotome; figures 44 to 47, from the 

 level of the twenty-third myotome; figures 48 to 50 from the 

 level of the twenty-eighth myotome of the three oldest embryos; 

 figures 51 to 53, from the level of the thirty- third myotome of 

 the same specimens; figure 54, from the level of the unsegmented 



