488 EDGAR J. ALLEN. 
between the unipolar and bipolar type of cell. It may be 
regarded as a bipolar cell in which the two poles have come to 
lie closely together, or as an unipolar cell the single process of 
which immediately bifurcates. It is interesting to note that 
in the lobster embryo all the cells of the cesophageal ganglion 
(fig. 2, r) which have stained, are of this type. 
His! has shown that the cells of the spinal ganglia of human 
embryos are at first bipolar, giving off one nerve process to the 
cord, the other to peripheral parts. In later stages the body 
of the cell becomes excentric to the axis passing through the 
two fibres, and by degrees a single process with a T-shaped 
fibre is formed. V. Lenhossék* has found both bipolar cells 
and unipolar cells with a T-shaped fibre in Pristiurus embryos, 
and discusses the transition of one form into the other. 
Ramon? also notes the fact that in embryonic birds the cells of 
the spinal ganglia are all bipolar, whereas in adult mammals 
they are unipolar with a bifurcating fibre.* 
In addition to those already mentioned, fibres from several 
other sources enter the anterior nerve. Some fibres pass down 
from the azygos nerve (hence probably from the gastric 
ganglion) directly into one or other of the anterior nerves. 
Such fibres are represented (c) in fig. 3, and they are also in- 
serted in fig. 1. A fibre, which passes from the dorsal nerve of 
one side to that of the other (fig. 1, m), has also been observed 
to give off a branch which passes down one of the ventral 
nerves. 
Lastly, the esophageal ganglion is connected directly with 
the brain by an anterior median nerve (ant. med. n., figs. 1, 
3, 4) which Huxley describes as “ the anterior continuation of 
the azygos nerve.”* Fibres passing from the brain along this 
1 His, “ Histogenese und Zusammenhang der Nervenelemente,” ‘ Arch. 
Anat. u. Entw. gesch. Anat. Abt.,’ Supp. Bd., 1890. 
2 'V. Lenhossék, “ Beobactungen an den Spinalganglien und den Riick- 
enmark von Pristiurusembryonen,” ‘ Anat. Anz.,’ 7, 1892. 
3 Ramon y Cajal, “Sur l’origine et les ramifications des fibres nerveuses 
de la moelle embryonnaire,” ‘ Anat. Anz.,’ 5, 1890. 
4 Compare also Retzius, ‘ Biol. Untersuch.,’ neue Folge iv, 1892. 
§ I have only observed one such nerve (cf. Huxley, ‘ Anat. Inv.,’ p. 287). 
