6 F, L. LANDACRE 
THE 10 MM. EMBRYO (GENERAL) 
Owing to the fact that the nerve components of the adult 
Lepidosteus have not been worked and that there are some pecu- 
liarities in the size and arrangement of the ganglia and nerves, it 
will simplify the description of the placodes to give a general 
account of structures and to give a rather complete account of 
the gangla and nerves of some stage, for which I have selected 
the 10 mm. embryo. 
This stage represents an intermediate condition in theforma- 
tion of the epibranchial ganglia; the placodes being completely 
detached from the epidermis in the geniculate or VII ganglion, 
in process of formation in the IX or petrosal and first two epi- 
branchial ganglia of the X, while the two remaining epibranchial 
placodes of the X are not yet present, there being only three well 
defined gill bars. 
The general form of the anterior end of the body is quite char- 
acteristic as shown in fig. 1. The anterior end of the head has 
a sharp downward flexure beginning in the region of the infun- 
dibulum and ends in a prominent sucking dise. Directly over 
the posterior end of the infundibulum lies the mesencephalon 
which marks the highest point on the dorsal surface of the body. 
The mouth is open and the pharynx has a cavity to a point 
just posterior to the third gill. Neither teeth not taste buds 
can be detected in the oral cavity or pharynx. 
No cartilages are present at this stage, although the primordia 
of both cartilaginous and muscular structures can be identified 
in condensed masses of mesoderm. 
The olfactory capsule has no cavity and is attached to the 
ectoderm throughout almost its whole length. Both roots of 
the olfactory nerve are well formed, that is, fibrillated and there 
are a number of loose cells lying around the nerve that probably 
develop later into the ganglion of the nervus terminalis. The 
optic vesicle is large, and elongated slightly in its anterior-pos- 
terior axis. 
The auditory vesicle is a simple sae slightly elongated from 
anterior to posterior and has a short ductus endolymphaticus. 
