289 
in the first 16 segments. This was in a3 mm fish. In one of 4 mm 
115 cells, contained in 21 segments, were present, while in one of 
51/, mm there were 129 cells which extended through 23 segments. 
If these fishes were the young of Bothus they would possess upon 
reaching maturity about 450 of these cells which would extend in a 
single row through the first 26 segments. This increase in the number 
of cells is probably due to a process of differentiation out of the sur- 
rounding embryonic cells, especially those caudad of the row. The 
neurites of these cells were first seen in the 4 mm specimen, where 
they were in a very early stage of their development. They passed 
ventrad in the cord toward 
the location of the future 
fibre bundle which began to 
make its appearance in the 
largest specimens obtained. 
These larvae were upright in 
position, even those in which 
the eye was rotated. 
Fig. 3. Transversejsection of spinal 
end of a larval Bothus 6 mm long. 
Two giant cells of the double row in 
median dorsal] fissure. 
Pleuronectes americanus. 
The second larvae examined were those of Pleuronectes 
americanus and were surface-swimming and upright in position, 
although the larger ones had rotated the eyes. In longitudinal sections 
the cells were carefully counted and the average number in each of 6 
specimens was 69, or a few less than in the adult. These specimens 
were 6 mm long, but slender, and in about the same state of deve- 
lopment as the preceding species. 
Summary. 
The adults of all the species examined possess this giant ganglion 
cell apparatus. It consists of a row of very large nerve cells in the 
median dorsal fissure and of their neurites, which pass caudad in the 
neuron and form an isolated fibre tract on the mesal side of each dor- 
sal horn. 
The giant cells of this apparatus are the first ganglion cells to 
become differentiated in the embryo flat-fish. They are not preceded 
or accompanied by a really transient apparatus and are the only large 
nerve cells that appear in the dorsal median fissure. 
