4 Journal of Comparative Neurology. 



myelic structures into the oblongata, and it is in the dorsal por- 

 tions, as usual, that the change is most marked. In a section 

 of typical myel the ventral cornua are narrow and extend latero- 

 ventrad ; dorsad of the myelocoele is a region of cinerea from 

 which the delicate dorsal cornua extend terminating in swellings 

 composed of amyelinic fibers and "ground substance" with 

 numerous small cells interspersed. Surrounding these on the 

 dorsal, mesal and lateral sides are fine closely aggregated myel- 

 inic fibers. The ventral tracts are composed of coarser fibers 

 with the characteristic Mauthner fibers ; the lateral tracts 

 are formed of fibers, in general, intermediate in caliber between 

 those of the dorsal and ventral portions. 



As the oblongata is approached, the dorsal horns enlarge, 

 gaining a size three or four times that characteristic of the 

 myelic portion (Figs. 6 and 13). At the same time the typic- 

 ally small myelocele enlarges and assumes a subtriangular 

 section ; the sulci forming the angle sextending toward the dor- 

 si-meson and the ventral cornua. The larger part of the dorsal 

 fibers disappear and just caudad of the metatela a concentration 

 of fine fibers on the dorso- and ventrolateral sides of the cornua 

 mark the first recognizable appearance of the spinal Vth tract. 

 At this level the dorsal cornu and the gelatinosa rapidly disap- 

 pear. (Fig. 15). 



Near the caudal end of the metatela, a lateral sulcus ap- 

 pears, and dorsad of it the first appearance, as such, of the fas- 

 ciculus communis (lobus vagi). (Fig. 15). 



Increase in size of the fasciculus communis tract and migra- 

 tion ventrad of the spinal Vth tract give the former for a 

 short distance a dorsal position. Soon, however, there appears 

 dorsad of the spinal Vth tract and the fasciculus communis an 

 area of fibers and intermingled small cells, which increases rap- 

 idly in extent and soon becomes capped by a layer of amyelinic 

 substance, the cerebellar crest (cerebellarleiste of Goronowitsch), 

 a caudal continuation of the molecular layer of the cerebellum 

 (Figs. 16, 17). The change in the morphology of the oblon- 

 gata from this point cephalad is simply in the increase in size of 

 this, the acusticum, displacing farther ventrad the spinal Vth 



