62 CHARLES BROOKOVER 



this stage, than formerly, between the placode and the outer 

 wall of the brain. Mesenchyme accompanied by capillaries and 

 blood corpuscles has pushed into this space. An outer zone of 

 nerve fibers has appeared just inside the brain wall and extends 

 from a point anterior of the olfactory nerve to some distance 

 posterior of it. 



Three hours later there are many more fibers in the olfactory 

 nerve but no nuclei in its course. In one preparation of this 

 stage a very small cavity can be made out near the center of the 

 placode. This cavity is not connected with the outside, but lies 

 a little nearer the lateral side of the placode opposite the slight 

 depression already mentioned as occurring in the epidermal layer 

 of the ectoderm. The cells of the mesial side of the placode have 

 assumed their columnar epithelial shape with nuclei at some dis- 

 tance from the lumen, as in the adult ; but those on the opposite 

 side of the lumen, where the external opening will appear later, 

 are more rounded and irregular in arrangement. From this stage 

 onward it is readily seen that the nasal capsule is continuous 

 at its border just under the epidermis with the inner layer of the 

 ectoderm in which taste-buds arise a little later. It can be said 

 then, that at this age, some sixteen hours before hatching, the 

 cells on the median side of the placode have assumed a more or 

 less definite character of neuro-epithelium and it is probable that 

 the olfactory nerve is composed of definite neurofibrils. The 

 fuchsin stain shows a plainly fibrillar structure. 



Other stages are about the same until six or seven hours before 

 hatching, when in horizontal sections two bundles of fibers can 

 be seen converging to a union just before entering the brain 

 wall (fig. 3). The anterior one is slightly larger and is probably 

 the older, as the earliest fibers are generally seen near the anter- 

 ior border of the placode. Some three or four cells are seen among 

 the olfactory fibers just as they leave the nasal capsule. Mesen- 

 chyme cells having capillaries and blood corpuscles in their 

 midst, crowd near the nerve but in many of the preparations can 

 be distinguished by their staining reactions from the cells inside 

 the nerve. The two branches of the nerve can be followed well 

 in among the nuclei of the nasal capsule. There is no external 



