ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
19 
cells on the surface of the spinal cord or brain, and in the interior 
of the latter ; it is only later that a nucleated and cell-containing tissue 
grows into the white substance. 
(6) As the central nerve-fibres have no nucleated sheaths it is 
clear that Schwann’s cells do not form the axis-cylinder and medulla. 
(7) The outgrowth of nerve-cells (neuroblasts) into the axis- 
cylinder has been seen, and is not difficult to demonstrate. 
(8) The method of Golgi shows that in young embryos the nervous 
processes are formed from cells, which belong partly to the medullary 
plate and the adjoining ectoderm, and partly to the peripheral ectoderm. 
(9) All the very fine peripherally distributed nerve-fibres such as 
those of the cornea and others have no nuclei, and show us that the 
axis-cylinder may innervate a wide area by simple longitudinal growth 
without any part being taken by foreign cells. The same is shown by 
Golgi’s so-called sensory cells of the second order. 
(10) Finally, the processes of nerve-regeneration show that this 
is effected solely by the axis-cylinder. 
(11) Prof. Kolliker now finds that the cells which he thought formed 
olfactory fibres are secondary and mesodermal structures. 
(12) He concludes that all nerve-fibres are direct processes of nerve- 
cells. 
Development of Optic Nerves of Vertebrates.* — Mr. K. Assheton 
points out that there are now two very distinct theories as to the origin 
of the optic nerve ; that which is generally held in this country is that 
which was thus expressed by Balfour : — “ The fibres of the optic nerves 
are derived from a differentiation of the epithelial cells of which the 
nerve is at first formed ; ” in Germany, however, His, W. Muller, 
Mihalkovics, and Kolliker agree that nerve-fibres are outgrowths from 
nerve-cells, and that sensory fibres of sense-organs grow inwards from 
the sensory epithelium of the sense-organ to the central nervous system. 
The author has studied the development of the optic nerve in the frog 
and in the chick, and he comes to the conclusion that the optic stalk 
takes no part in the formation of the nervous parts of the organ of 
sight. This optic stalk becomes broken down and the cells composing 
it are separated from one another, partly by the mechanical stretching 
due to the growth of the optic nerve, and partly by the growth in 
between the several cells of the nerve-fibres. The nerve-fibres of the 
optic lie along the posterior border of the stalk, and are at first entirely 
outside it ; but, on the breaking down of the stalk, some of the nerve- 
fibres grow in between the cells. The great majority of fibres forming 
the optic nerve arise as outgrowths from cells in the retina, and grow 
towards and into the brain. Cajal has discovered certain fibres which 
would seem to grow from the central nervous system to the retina, but 
these Mr. Assheton has not been able to find. The nerve-fibres pass 
over the ventral edge of the optic cup and thereby cause the formation 
of the choroidal fissure ; at this point there is no proliferation of cells. 
The author remarks that he has never seen the suggestion that the 
fissure represents a stage in the evolution of the eye, being, of course, 
ignorant at the time of writing that Prof. Biitschli was about to make a 
* Quart. Journ. Micr. Sci., xxxiv. (1892) pp. 85-104 (2 pis.). 
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