276 G. E. COGHILL 



region but not in the region from which the olfactory nerve of 

 later stages arises. In the early flexure stage the external con- 

 cavity has deepened slightly. The cells of the olfactory epi- 

 thelium have become highly columnar and heavily pigmented 

 peripherally. The organ has come into rather close contact with 

 the brain in the rostral region, but no direct connection can be 

 made out between them. In the coiled-reaction stage the external 

 concavity has become a shallow pit with well defined margins, 

 but the depth is not equal to the diameter of the aperture. There 

 is now very intimate contact between the olfactory epithelium 

 and the brain at the point where the olfactory nerve is about to 

 appear, but no fibers have been observed entering the brain in 

 this stage. In the transition from this to the early swimming 

 stage the olfactory pit has deepened considerably but has not 

 yet dilated perceptibly within the nares. The olfactory nerve 

 has now become well established. It arises from near the rostral 

 margin of the olfactory epithelium, where the latter is in actual 

 contact with the brain. Silver impregnations indicate that the 

 fibers arise from neurones situated near the base of the nerve, 

 though the impregnations of the cell bodies has been very imper- 

 fect and unsatisfactory. In frontal sections (parallel with the 

 longitudinal axis of the embryo) the fibers appear running both 

 caudad and rostrad within the brain into a zone of highly differ- 

 entiated neurones which, presumably, are the mitral cells. There 

 is a cell-free area here which obviously represents the glomerular 

 zone. 



6. Summary 



A. THE NON-MOTILE STAGE 



The ophthalmic and Gasserian ganglia are both connected with 

 the skin at their distal ends (figs. 1, 7 to 12). No fibers can be 

 clearly traced from them to the skin. Their root fibers, however, 

 reach the brain. Large cells, having the characteristics of the 

 Rohon-Beard cells of the spinal cord, situated near the entrance 

 of the trigeminal root, send processes out into this nerve (figs. 

 20, 21). The general cutaneous ganglion of the vagus (jugular) 



