428 Prof. G. Retzius. Principles of the [June 15, 
and 1892, that no mode of termination of that kind exists, that the nerve 
terminations find their way into the taste-buds, and copiously ramify 
there among the taste-cells without coming into direct connection with them. 
As regards the olfactory organ, it could be shown clearly and distinctly by 
the Golgian method that the olfactory cells of the mucous membrane send 
out a central process which makes its way into the glomeruli of the 
olfactory bulbs, and there copiously ramifies among the ramifications of one 
or more other axons proceeding from the central nerve-cells, forming with 
them the glomeruli. That has been shown already by Golgi, and was 
confirmed by Cajal, Van Gehuchten, myself, and Kolliker; the four last 
named did not discover any direct connection between the nerve-fibres that 
proceed from the centre and those from the periphery, though that there is 
none is often difficult to prove for certain by reason of the exceedingly 
complicated convolutions of the fibres. 
The sensory cells of the olfactory organ are to be regarded as a species 
of peripheral nerve-cells. They must also be regarded as corresponding 
fundamentally to those bipolar cells in the skin of the invertebrata already 
mentioned above. 
In 1881 to 1884 I had shown that in the macule and criste acustice of 
the auditory organs the nerve-fibres enclose the lower ends of the hair cells 
by means of, calyx-shaped extensions, and even send out fine fibrils that 
proceed in an upward direction on the surface of these cells. Here the 
matter proved more complicated, for by the Golgian method I succeeded. in 
1892 in proving definitely that the terminations of the auditory nerve come 
to an end with free ramifications among the hair cells, and, to some extent, 
find their way up to the surface of the epithelium. More recently Cajal has 
shown by his new silver method that both modes of termination, calyx 
formation and free ramification, exist side by side in criste acustice. | 
It is in any case plain that sensory cells of the auditory organ (the hair 
cells) may in principle be compared with those of the gustatory organ. 
Their development makes manifest, too, that they do not send out any fibres 
which proceed towards the centre, but that the bipolar nerve-cells situated 
in the ganglion acusticum dispatch their peripheral processes to the sensory 
cells in the epithelium. 
By reason of these circumstances, constituting as they do an essential 
difference, I here distinguish sensory cells such as those of the olfactory 
organ, which I call primary, from sensory cells such as those of the gustatory 
and auditory organs, which I call secondary. The former are, of course, 
a species of peripheral nerve-cells, the latter are not. 
I do not propose here to enter upon a discussion of the complicated 
