216 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



are proliferated from the region of encephalomere VI, tlie greatest pi-o- 

 liferation occurring, however, as in the case of encephalomere Y, in the 

 posterior part of the encephalomere. No previous observer has stated 

 that the cells of the ganglionic Anlage of the ninth nerve are proliferated 

 from encephalomere VI. However, that previous observers have seen 

 the prolifei-atiou of cells from this encephalomere is possibly shown bv 

 the fact that both Shipley ('87) and Kupffer ('94) have found in Petro- 

 myzon, between the Anlagen of the 7th and 9th nerves, a " weak 

 primitive acusticus, which soon vanishes." Hoffmann ('94) stated that 

 in Acanthias embryos with 32 to 35 somites, a new outgrowth appears 

 between the facialis and the glossopharyngeus, which to all appearance 

 is a rudimentary and early aborting segmental nerve. Although Hoff- 

 mann published no figures, I infer from his description that this out- 

 growth, or rudimentary nerve, is that portion of the neural ridge which 

 is proliferated from the region of encephalomere VI. I am at least able 

 to say positively that no other outgrowth of cells takes place just pos- 

 terior to the Anlage of the acustico-facialis. In the phenomena pre- 

 sented by this outgrowth Hoffmann finds the chief support for his 

 contention that the Anlagen of cranial nerves arise as paired segmental 

 outpocketings of the neural tube, corresponding to, or comparable with, 

 the outgrowth of the eye vesicles. He figures diagrammatically the out- 

 growth of the neural crest in the region of the glossopharyngeus Anlage 

 as an outpocketing of the dorsal wall of the neural tube possessing a 

 lumen continuous with that of the tube. At no time do I find evidence 

 of a lumen between the neural-crest cells, although in later stages the 

 nuclei in the VII and IX ganglionic Anlagen tend to take a peripheral 

 position. 



At a stage with 26 or 27 somites (Plate 3. Fig. 12) the thalamic por- 

 tion of the trigeminus Anlage is no longer continuous dorsally with the 

 posterior portion of the Anlage, the cells of which come to lie in the 

 region of constriction between midbrain and hindbrain. The thalamic 

 portion extends from the constriction between primary forebrain and 

 midbrain toward the eye vesicle, just behind which it unites with a line 

 of cells, ectodermal in origin, which extends along the dorsal border of 

 the eye close to the superficial ectoderm. Some of the cells of the 

 trigeminus Anlage now extend into the mandibular arch, and have there 

 come to surround the mandibular mesoderm. 



A displacement of the cells of the Anlage of the acustico-facialis and of 

 the glossopharyngeus has begun at this stage. This is clearly to be ac- 

 counted for by the invagination of the auditory epithelium, which is now 



