Miscellaneous. 153 



same fibre presents several points at which there are these bodies. 

 Where they are large (having a diameter of from 00 12 to 0020 

 mill.), the nerve upon which they occur is large. Besides the large 

 bodies, there are smaller ones, only OOOG 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 ^ mill. 

 At the origin the trunk is not more than -A— ^V >"ill- i" thickness. 

 At 1^ mill, from the origin this nerve divides into three branches, 

 each of which is ^^-^ mill, in diameter. At -j^ mill, further on, each 

 branch again^divides, and each division is about ^ mill, in thickness. 

 At j>^ mill, further, there is another division into several filaments, 

 each ji^i; mill, in diameter. From this the residue of the divisions 

 has still to traverse a distance oi-^, mill, to the lower margin of the 

 head. The thickness of the nerves diminishes to ^hu ™ill' l «"d from 

 these very delicate tilnmcnts the terminations arise, altnough some have 

 already originated from the anterior trunks. The final terminations 

 are very short branches — Utile cylinders, which issue from the two 

 sides of the terminal trunks in great quantities, and which measure 

 „^ 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 tibres may be seen, 

 but this is rare, lint at the extremities of the cylinders we see issu- 

 ing from their midst the terminal nerve-fibres, which arc transparent, 

 greyish, without the least trace of granules, and without a sheath. 

 It is only with a power of 4 JO to JOO diameters that they can be 

 clearly seen. Hut in order to trace their ultimate distribution a 

 power of 7.')0 diameters is necessary. The terminal fibre, a cylin- 

 draxis, measuring 00005, 00008, and O'OOl mill, in diameter, di- 

 vides afresh, and becomes a little dilated on issuing ; from these 

 inflations, which contain neither nucleus nor gramdes, issue fibres 

 which run to other small inflations, and so on. In this manner is 

 formed a network, which I 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 cyUnders; 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. 



