ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
521 
Freud in the spinal medulla of the lamprey, and the cells of Eohon 
in the trout — cells whose axis-cylinder process enters by one branch 
at least into the posterior root. These are the true “ radicular dorsal 
cells.” (2) There are the dorsal cells of the spinal medulla in Tropi- 
donotus matrix and Salamandra maculosa , cells completely independent 
of the posterior roots, whose axis-cylinder process becomes a constituent 
fibre of the white matter of the cord. These are the true “ medullary 
dorsal cells.” 
Tubular Enamel.* — Mr. C. S. Tomes discusses the peculiarity of the 
enamel in Marsupials. Except in the wombat, it is, as Sir John Tomes 
pointed out, freely penetrated by tubes, which enter it from the dentine, 
and are continuous with the dentinal tubes at the junction of the two 
tissues. The same condition occurs sporadically in other mammals, 
e.g. jerboa, shrew, and Hyrax. 
The author concludes that the special cells of the enamel organ 
(ameloblasts) do not themselves calcify ; that they each furnish from 
their free ends outgrowths or processes which are continuous with their 
own plasm, and which may be traced through the entire thickness of 
young enamel ; that one ameloblast furnishes the whole length of an 
enamel prism ; that the fibrillar outgrowths, previously more or less 
correctly described in other enamels, but not adequately appreciated, do 
calcify from without inwards in such a manner that an axial canal is 
left uncalcified ; that the canals of marsupial enamel are in the centres of 
the prisms, and not, as supposed by Yon Ebner, in the interstices of 
the prisms ; and that towards the completion of the full thickness of the 
enamel, the central axis is no longer left soft, but the whole calcifies 
into a solid prism. Briefly stated, the author’s belief is that “ all enamels 
alike are formed by the centripetal calcification of fibres furnished by 
the ameloblasts, and that tubular enamels are nothing more than the 
perpetuation of a stage which is passed through, though only for a 
brief period, by every solid enamel prism.” 
Nervous System of Ammocoetes.f — Herr F. Mayer distinguishes in 
the cerebrum of Ammocoetes tbe olfactory lobe, the main ganglion (corpus 
striatum), and the cortex, each with typical fibre connections. In the 
thalamencephalon and mesencephalon most of the typical tracts are 
also demonstrable. The olfactory tracts may be followed to the cortex, 
and (as taenia thalami) to the superior commissure. They are thus 
related to the thalamus and hypothalamus, and through the ganglion 
habenulae and Meynert’s bundle to the cerebellum. Muller’s fibres 
are neurites of colossal ganglion cells which place the optic region 
and that of the nerves following in connection with the spinal cord. 
The epiphysis is a functioning organ in nervous connection with the 
mesencephalon. Most of the ganglion cells, and with particular dis- 
tinctness those of the corpus striatum and the hypothalamus, have an 
“ epithelial process ” which extends to the central cavity. Even in late 
stages of development some of the neurites in the central nervous 
system end freely in a point. In most of the fibre-tracts and com- 
missures there are dendrites as well as the neurites running in the 
opposite direction. 
* Proc. Roy. Soc. London, Ixii. (1897) pp. 28-30. 
f Anat. Anzeig., xiii. (1897) pp. 649-57 (1 pi.). 
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