Miscellaneous. 158 
same fibre presents several points at which there are these bodies. 
Where they are large (having a diameter of from 0°012 to 0:020 
mill.), the nerve upon which they occur is large. Besides the large 
bodies, there are smaller ones, only 0°006 mill. in diameter. Large 
and small loops are found upon the same nerve ; but the small loops 
occur sometimes before and sometimes beyond the large ones. The 
terminations of the nerves are not in these loops. 
In the Branchiostoma we have the great advantage of being able 
to examine the nerves from their origin to their extremities. The 
space traversed by them in the head is very small. Thus, if we take 
one of the three nerves which issue from the anterior extremity of the 
central nervous system, and which, running from above downwards, 
distributes itself in the lower part of the head, its length from its 
origin to its termination in the lower margin of the head is only 4 mill. 
At the origin the trunk is not more than =)—,/5 mill. in thickness. 
At 5% mill. from the origin this nerve divides into three branches, 
each of which is =, mill. indiameter. At 5% mill. further on, each 
; 5 
branch again@divides, and each division is about J; mill. m thickness. 
At j< mill. further, there is another division into several filaments, 
each ++, mill. in diameter. From this the residue of the divisions 
has still to traverse a distance of ;% mill. to the lower margin of the 
head. The thickness of the nerves diminishes to 51, mill.; and from 
these very delicate filaments the terminations arise, although some have 
already originated from the anterior trunks. The final terminations 
are very short branches—little cylinders, which issue from the two 
sides of the terminal trunks in great quantities, and which measure 
soo mill. in diameter and a little more in length. Up to this point, 
and including the terminal cylinders, the nerves have a transparent 
sheath with granular contents, which prevent our seeing the primi- 
tive nervous fibres which were readily detected in the roots of the 
nerves. In some places indeed something like fibres may be seen, 
but this is rare. But at the extremities of the cylinders we see issu- 
ing from their midst the terminal nerve-fibres, which are transparent, 
greyish, without the least trace of granules, and without a sheath. 
It is only with a power of 450 to 500 diameters that they can be 
clearly seen. But im order to trace their ultimate distribution a 
power of 750 diameters is necessary. The terminal fibre, a cylin- 
draxis, measuring 0°0005, 0°0008, and 0:001 mill. in diameter, di- 
vides afresh, and becomes a little dilated on issuing ; from these 
inflations, which contain neither nucleus nor granules, issue fibres 
which run to other small inflations, and so on. In this manner is 
formed a network, which [ at first believed to be terminal ; but some- 
times, on slightly moving the screw of the microscope, I have seen 
starting, from what I thought to be the end, other filaments which 
I could trace no further. 
I have said that the cylindraxes divide after issuing from the ter- 
minal cylinders; but the primitive fibres of the trunks must also di- 
vide. At the origin there are only from five to seven primitive fibres. 
In forming the terminal cylinders, of which the number amounts to 
twenty, thirty, or even more, the primitive fibres must subdivide. 
