THE EPIBRANCHIAL GANGLIA OF LEPIDOSTEUS if 
The supra-orbital, sub-orbital, mandibular and body sensory 
lines are present (figs. 6, 7, 15) and well defined and the position 
of some of the lateral line organs can be determined, although 
they are not well differentiated and not all of them are present, 
even on the head. The epidermal thickenings (figs. 8 and 9) 
forming the caudal extension of the epibranchial placodes are 
present and extend from the point of origin of the epibranchial 
placodes of the VII and IX nerves. They can be distinguished 
histologically from the primordia of the lateral lines in both the 
VII and IX nerves, and in the case of the IX, lie at a lower level 
than the primordia of the lateral lines. 
The brain (fig. 1) at this stage shows a few of the peculiarities 
that are so pronounced in the adult Lepidosteus. The three 
primitive brain vesicles are still present. The telencephalon is 
not sharply separated dorsally from the diencephalon; in fact, 
except for arbitrary landmarks it is not possible to define the two 
divisions of the primitive prosencephalon. The epiphysis and 
dorsal sac are present at the boundary between the thin walled 
prosencephalon and thick walled mesencephalon, but the para- 
physis is not present and appears first in an embryo of 11 5 mm. 
as a definite evagination, although its position can be determined 
somewhat earlier as a thickened: area in the roof of the prosence- 
phalon. With the appearance of the paraphysis comes the ele- 
vation and broadening of the thin roof of the telencephalon which 
marks it off as distinct from the diencephalon. 
Ventrally the hypophysis is well developed and there is a marked 
flexure in the floor of the brain directly under the mesencephalon 
and directly over a point just anterior to the posterior end of 
the hypophysis. The pituitary body is well developed at this 
stage. The mesencephalon is well developed and rises consider- 
ably above the level of the roof of the brain and consists of a large 
flattened vesicle. which extends laterally over the anterior end 
of the metencephalon and has uniformly thick walls (figs. 6 and 
7). These lateral lobes of the mesencephalon completely con- 
ceal the anterior end of the metencephalon from the dorsal sur- 
face. The roof of the brain is of uniform thickness from the 
region of the epiphysis to the posterior end of the mesencephalon. 
