MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN, 307 



which has proved a fruitful source of misconception as to the 

 true nature and relations of the olfactory nerves, especially 

 as these latter are usually not recognised until they have 

 taken up this secondary position. 



The change to which I have just referred is well illus- 

 trated by fig. 11, a transverse section through the anterior 

 part of the head of a chick embryo at the eightieth hour. 

 The section shows the commencing cerebral hemispheres 

 [c. h.) growing upwards and outwards from the forebrain; it 

 also passes through the margins of the two olfactory pits 

 {olf.), and on the left side through the root of the olfactory 

 nerve (i) at its point of origin from the brain ; the figure 

 shows very clearly the effect of the appearance of the cerebral 

 hemispheres on the position of the olfactory nerves, and 

 shows further how the secondary connection of these nerves 

 with the hemispheres is acquired. 



Fig. 12 represents a section from the same series as fig. 11, 

 but taken a little further back, passing through the olfactory 

 pits {olf.) at their deepest parts. On the right side the 

 section passes through the distal portion of the olfactory 

 nerve (i), which is seen to be in continuity with the bottom 

 of the olfactory pit. 



In figs. Hand 12 the olfactory nerve has the same histo- 

 logical character as in fig 10 ; it is, however, relatively, if 

 not indeed absolutely, smaller than at the earlier period. 

 The figures further show clearly that there is as yet no trace 

 whatever of an olfactory lobe. 



I have elsewhere^ given figures and description of the con- 

 dition of the olfactory nerves at the ninety-third hour in the 

 chick, at which date, excepting a general increase in size, 

 their condition differs but little from that at the eightieth 

 hour. 



Figs. 7 and 8 represent longitudinal and vertical sections 

 through the anterior part of the head of a chick embryo to- 

 wards the end of the sixth day of incubation. As the olfac- 

 tory nerve did not lie exactly in the plane of section it has 

 been necessary to figure two sections, of which the more 

 superficial one (fig. T) shows the greater part of the length 

 and the peripheral distribution of the olfactory nerve ; while 

 the second section (fig. 8), taken at a slightly deeper level, 

 shows the root of origin of the nerve from the brain. The 

 olfactory nerve, which is still short, presents a proximal gan- 

 glionic swelling at its point of origin from the hemisphere, 

 seen best in fig. 8 ; along the greater part of its length the 

 nerve consists of very elongated fusiform cells, with a few 



' Loc. cit., p. 20, and Plate II, figs. 17—19. 



