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completely resembling those of the nuclear stratum in the olfactory 
epithelium of the nasal cavity; i. e. they possess rounded nuclei 
(6—7 » x 5p) surrounded with very scanty protoplasm prolonged into 
processes at either end. The highest columns measure about 300p. 
8. The Floor. The following structures may be traced from roof 
to floor: 
1) the layer of ordinary cylindrical epithelial cells, which are now 
only 15p high, and bear short cilia: between the bases of these are 
wedged small rounded cells forming rarely more than one layer; these 
rest on 
2) the capillary plexus, which is directly continuous with that 
mentioned above : the rest of the mucosa is occupied by 
3) the abundant pigment cells, which spread out at junction of 
roof and floor to surround the cellular columns in the mode described 
above. 
Structure of the parts in the embryo examined. 
The following points are worthy of remark. The neuro-epithe- 
lium in the nasal cavity is as high as in the adult, but the glands of 
Bowman are not yet developed. The highest of the cellular columns 
in Jacobson’s Organ are from 150—180p, while the neuro-epithelium 
(inside the capillary plexus) is from 56—67y high, of which 28w still 
belongs to the superficial stratum. The nuclear stratum (inside the 
plexus) is thus much higher in the embryo than in the adult, while 
the cellular columns are not yet so high, and are separated by a some- 
what more abundant mucosa. 
From the above data I conclude that the cellular co- 
lumns in the roof of Jacobson’s Organ are outgrowths of the 
nuclear stratum of its neuro-epithelium, the polygonal form 
of which has been determined by the meshes of the capillary 
plexus through which the outgrowths have taken place, 
and that in the course of development more and more of the 
cells of the nuclear stratum have been pushed outside the 
boundary formed by the capillary plexus, till eventually 
little but the superficial stratum is left inside that boun- 
dary. 
The cellular columns push before them the basement membrane, 
only traces of which (opposite the nodes in the plexus) are to be found 
in its original position inside the plexus. 
I have purposely used the term ,nuclear stratum‘ rather than , layer 
of olfactory cells‘, because I doubt whether the superficial processes 
of the cells in the deeper layers ever do reach the surface, and thus 
fulfil the definition of an olfactory cell. On the other hand their deep 
