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neiiropore to the olfactory epithelium. This locates its point of origin 

 on the apex of the primary fore- brain. Its earliest history is difficult 

 to clear up. I have given much attention to sections of embryos from 

 6 to 8 mm long and I have repeatedly observed that there exists a 

 cellular connection between the olfactory plate and the brain-wall as 

 described by Hoffmann ('96). The new nerve has at first a fusion 

 (placode) with the thickened surface epithelium, located just above the 

 shallow depression that marks the beginning of the olfactory pit. This 

 connection between surface epithelium and brain-wall, consists of a 

 group of closely packed cells in which I have failed, in this early 

 stage, to recognize fibers. In embryos about 10 mm long, however, 

 fibers are to be seen extending from the sides of the neuropore to 

 the olfactory epithelium. These I take to be fibers of the new nerve, 

 but have not been able to satisfy myself as to the position of the 

 neuroblasts that give rise to them. The neural crest is disappearing 

 in this region, and my observations incline me to the view that the 

 neuroblasts of the fibres of the new nerve are, at least partially, de- 

 rived from the cells of that structure. 



For a short time there is a single fibrous connection on each 

 side between the brain-wall and the nasal epithelium. Very soon a 

 second similar connection, more lateral in position, is established be- 

 tween the brain-wall and the olfactory pit. The two connections are 

 entirely independent as regards their union with the brain wall. The 

 earliest of these fibrous tracts to be established is the new nerve, the 

 later one the main olfactory. The two are present in embryos 13 mm 

 long^) (and perhaps in still smaller ones). 



By the time the embryo has reached a length of 16 mm the two 

 independent brain connections are clearly differentiated. The point of 

 union of the main olfactory with the brain is more lateral in position, 

 and is composed of two roots; that of the new nerve is on either 

 side of the neuropore, and shows a ganglionic enlargement. 



1) The length is, of course, no sure criterion as to its age. Those 

 who have compared a large assortment of embryos of any animal, must 

 have been impressed with individual variations. Some embryos are 

 longer than others which are clearly more advanced in development, 

 and there is variation as to the number of gill- clefts broken through 

 on the two sides of the body, as well as in other anatomical landmarks. 

 The difference between embryos 10 mm and 13 mm is slight, and in indi- 

 vidual cases the longer one might be the younger. The chief point is 

 that my sections show stages in which the new nerve is present and 

 the olfactory not yet established. 



