The Reactions of the Vertebrate Embryo to Stimulation etc. 549 



the inner border of the cranial end of the myotome split up, some 

 fìlaments passing dorsally, others vcntrally, but only very occa- 

 sionally do any seem .to penetrate between the nuclei. Beginning 

 with the sections where the ventral roots become prominent several 

 important topographical chauges are noticeable. In the first place 

 the fibril bundles which migbt possibly be said to beloug to the 

 Vagus bave din» ini shed greatly in numbers so that only a few 

 straggling filaments remain to mark their point of exit frora the 

 cord. The second striking feature is the corresponding decrease in 

 the number of neurofibrils connected with the prominent latero- 

 ventral group of cells to which reference has frequently been made. 

 The comparatively few fibrils which stili continue to leave the 

 medulla at the spot corresponding to the exit of the true Va- 

 gus roots are now directed obliquely dorsal, instead of bending 

 sharply and passing outwards at right angles t9 the medulla, as 

 was the case in the sections through the areas lying above or in 

 front of this this piane. Probably the^e filaments belong to the Vagus 

 and are not the dorsal roots of s[)inal nerves. Gradually as the 

 cranial portions of the spinai ganglia come into the field the contrast 

 between the ventral horns with their countless fibrils and the 

 dorsal part of th^ cord conta ining scarcely a single fibril is very 

 great. Within the cranial portious of the spinai ganglia it is stili 

 impossible to detect the presence of any neurofibrils but as one 

 l)asses caudally these gradually augment in nnmber as the accumu- 

 lation of cells increases, until the total number of fibril bundles 

 contained in these structures may be said roughly to equa! those 

 in the ventral roots. The spinai ganglia bundles forni with those 

 from the ventral roots a plexus just in front of the ganglia, from 

 which point one may follow the fibrils creeping along the inner 

 border of the myotome almost to its ventral end. There is a niarked 

 disproportion in number between those in the ventral as compared 

 with the few that leave the dorsal part of the ganglia. 



An exceediugly important point is in reference to the number 

 of fibrils passing to the myotome. These are generally finer fila- 

 ments given off from the main truuk of the Ramus nervi spinali» 

 ventralis, which bend sharply at right angles as they approach the 

 inner surface of the myotome, where, at this stage, they generally 

 end abruptly, and only in rare instances, in cross sections, are they 

 seen to penetrate among the nuclei. The entire volume of the 

 ventral roots is apparently proportional to the development of the 



