468 EDGAR J. ALLEN. 
but as it showed no sign of breaking up or ending there, it 
probably goes to Abd. 6. This is rendered almost certain from 
the fact that the element agrees in other respects with the 
members of the series now being described, which have been 
actually traced all the way. On its way down the cord, the 
fibre gives off collateral branches to the neuropile of the dif- 
ferent ganglia. 
A (a) Th. I1—The cell of this element lies anteriorly 
in the lateral ganglionic mass of Th. I. The fibre gives 
rise to a figure of very characteristic shape in Th. I and II, 
sending off a peculiar branch, which passes downwards on the 
same side as that on which the cell is situated as far as Th. II, 
where. it turns inwards towards the centre of that ganglion. 
The main fibre after passing to the opposite side turns down 
the cord, and has been traced to Abd. 6. The actual ending 
in Abd. 6 has never stained. The fibre gives off collateral 
branches in the ganglia through which it passes. 
A (a) Th. I1.—The cell lies at about the centre of the lateral 
mass of ganglion cells. The fibre gives off an upward and 
downward branch to the neuropile of the same side, then 
passes across to the opposite side, and runs down the cord. 
It gives off collateral branches, and has been traced to Abd. 6. 
A (a’) Th. II.—This is an element starting from a cell in the 
ventral median mass of ganglion cells of Th. II. The fibre 
passes upwards and outwards to Th. I, where it decussates and 
then runs down the cord on the opposite side. It has been 
traced to the end of the thorax, but does not appear to stop 
there. It has collaterals, at least during the upper part of its 
course. 
A (a) Th, III. | —These fibres are similar in their relations 
A (a) Th. V. to A (a) Th. II, but they have only been 
actually traced to the end of the thorax. 
It may be explained here that the great difficulty in tracing 
fibres from the thorax to the abdomen of lobster embryos is 
due to the abdominal flexure. It is impossible to straighten 
out the abdomen without such injury as interferes with the 
staining, or causes it to disappear. Hence it is only when 
