112 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the whole is to be considered a complex sensory organ, analogous to the 
olfactory organ of Vertebrates in its intimate relation with the brain. 
Retzius shows that the sensory fibres of the ciliated groove are processes 
from bipolar cells of this region. The fibres of the fourteenth pair of 
nerves are the processes of cells similar in form and position to the 
bipolar cells of the thirteenth nerve. 
6. VENTRAL NERVE Corp. 
The structure of the ventral nerve cord has been well described for 
Lumbricus by Friedlander (’94), and Hatschek (89-91) has given a 
good figure of a transverse section of the ventral cord of Sigalion. Most 
writers, however, have not succeeded in preparing the ventral cord so as 
to show clearly that the connectives consist wholly of longitudinal fibres. 
There is nowhere in the ventral cord a neuropil in the sense of that 
which is found in the brain. There are small masses of fibrillations in 
the ganglia, of course, but they simply fill up the interstices between the 
fibres, and never occur in masses large enough to produce the punctate 
appearance peculiar to the neuropil of the brain. 
The paucity of nuclei among the fibres of the cord will not permit one 
to regard the fibre sheaths as composed of the expansions of non-nervous 
cells. In the decapod Crustacea the fibre sheaths are nucleate, and in 
the case of the sheath of giant fibres the nuclei are so numerous that 
the sheath may be described as a flat endothelium. In Nereis, however, 
the sheath must be a product of the fibre itself. 
7. CENTROSOMES. 
Since Lenhossék (’95) announced the discovery of the centrosome in 
the adult nerve cells of the frog, there have appeared a number of papers 
describing similar structures in Reptiles (Buehler, 795), Cyclostomes 
(Schaffer, 96), Molluscs (McClure, ’96), and Worms (Lewis, ’96). Hei- 
denhain (’97) summarizes the evidence and gives a bibliography. Dahl- 
gren (97) describes what he calls a centrosome artifact in the spinal 
ganglia of the dog. This artifact, he says, is produced by the formation 
of a crystal of corrosive sublimate in the cell. In Nereis I find the 
best demonstrations of centrosomes in preparations that have been fixed 
in corrosive sublimate, but they also occur in preparations fixed in the 
osmic acid mixture of vom Rath. I think there is no reason for con- 
sidering the phenomenon an artifact in this case. I will simply call 
attention in this connection to two facts that were mentioned previously ; 
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