40 EDGAR J. ALLEN. 
fibre just described, and whether the originating cell lies inside 
or outside the central nervous system remains unknown. It is 
possible that we are here dealing with an element similar to 
the elements T in Th. vir and vii, the cell of which however 
has never taken up the methylene blue. 
Elements in which one ganglion cell gives rise to two or 
more fibres passing out of the ganglion by different nerve-roots 
have been described by Retzius in Aulastomum gulo.! In 
that case the two fibres traced passed out by the two roots 
of the ganglion in which the cell lay, whilst a third, whose 
ultimate fate was not determined, passed into the general mass 
of longitudinal fibres of the ganglionic chain. 
Misce,tnanzous Exements.—In Th. iv a system of 
elements occurs, which offers certain difficulties in resolving 
it into its component parts. It is found to stain either com- 
pletely or incompletely in embryos at almost all stages of 
development. Its position and relations to the neighbouring 
ganglia are shown in fig. 1 (U, Th. iv), whilst the details will 
be best seen in fig. 4. As may be gathered from the latter 
figure only two cells, belonging to corresponding elements of 
the opposite sides, have stained, but it appears to me to be most 
probable that the system contains several sets of elements, the 
cells of some of which have never taken up the methylene blue. 
The only clue which it has been possible to obtain as to the 
course pursued by the individual elements is the fact that in a 
number of preparations of late embryos (near hatching) the 
portion of the system represented in fig. 5 has been alone 
stained, and may therefore represent a single element. If this 
be the case, the course of the element may be described as 
follows (cf. fig. 1, U., figs. 4 and 5): Starting from a cell in 
the anterior portion of the lateral mass of ganglion cells of Th. 
iv, the fibre runs near the dorsal surface of the ganglion with 
an almost straight course inwards towards its centre, where it 
gives off a pair of branches (figs. 4 and 5, c.) which run down- 
1 Retzius, ‘ Biol. Untersuch. Neue Folge,’ ii, ‘ Zur Kenntniss des centralen 
nervensystems der Wiirmer.’ 
