186 G. E. COGHILL 



wider distribution, and in the association of the fibers with mesen- 

 chymal cells. 



6. The great majority of the fibers of this system pass out 

 of the cord into intimate relation with the ends of the myotomes. 

 There may be in the later stages an increasing number that pass 

 out to the skin over the middle portion of the myotomes or rela- 

 tively free from them. 



2. THE SPINAL GANGLIA AND THE LATERAL LINE ORGANS 



That the anlagen of the spinal ganglia have no perceptible 

 dorsal roots during this time has already been mentioned. Their 

 condition in embryos of the early-swimming stage may be illus- 

 trated by the section through the fourth ganglion in figure 27. 

 The cells here are closely crowded together into a compact group 

 with definite border, but there are no clearly differentiated neu- 

 rones among them. Comparison of the members of the series 

 that appear in an embryo of this age shows that the anlagen of 

 the ganglia which are situated in what may be termed the cervi- 

 cal and lumbar regions are larger than those in the mid-trunk 

 region and farther caudad. This is obviously in anticipation of 

 the sensory innervation of the limbs. 



With view to a description of the lateral line system in a future 

 paper the primordia of the lateral line organs of the head have 

 been projected upon figures 56 to 59 along with the primordia 

 of the organs in the trunk. Only a brief description of the dis- 

 tribution of these primordia in the trunk, however, will be given 

 here, since a knowledge of their distribution and various experi- 

 mental data have made it possible to eliminate the consideration 

 of the lateral line organs from the fundamental physiological 

 problems that constitute the real occasion for the anatomical part 

 of the paper. 



The appearance of the primordia as seen in section through the 

 skin is represented in figure 27, which shows one of the structures 

 at the level of the fifth or sixth myotome in an embryo of the 

 early-swimming stage. The primordia of the youngest embryos of 

 the series are not so clearly differentiated. 



