18 F. L. LANDACRE 
reaches almost to the ventral border of the optic vesicle. The 
region of the overlapping of neural crest and mesoderm is quite 
extensive, as indicated in figure 8. In the region just posterior 
to the eye the mesoderm is not completely covered by neural 
crest. In the region of the VII, IX, and X ganglion the ventral 
half of each ganglion overlaps the lateral mesoderm and lies 
lateral to it. 
In figure 9 from a stage 4 mm. long and ten hours older than 
the stage from which figure 8 was taken, the neural crest has not 
altered its relation greatly except that posterior to the eye it 
has grown ventrally and caudally into the mandibular bar and 
now almost covers, on the lateral surface, the primordium of the 
mandibular muscles. This extension reaches almost to the 
ventral limit of the body and far beyond the ventral limit of the 
V ganglion. In the region of the VII and X ganglia, the ventral 
extensions of the crest have moved into the hyoid and second 
true branchial bars reaching, at least in the case of VII, to the 
middle of the body and well beyond the ganglion-forming region. 
The crest in the region of IX shows little change. 
The lateral mesoderm in the pharyngeal region is now inter- 
rupted by three visceral pouches, but is otherwise unmodified 
so far as its extent is concerned. In the anterior head region 
the mesoderm has extended cephalad over the eye and ventrally 
at the same level so that now it lies between the dorsal border 
of the optic vesicle and the brain. The increase in the amount 
of overlap of neural crest is most marked in the regions of the 
VII and X ganglia. 
Figure 10 is taken from stage 12 of series III and is 43 mm. 
long and approximately twelve hours older than the stage from 
which figure 9 was taken. In this stage there are two striking 
changes in the extent of the neural crest noticeable in the anterior 
head region. The first is progressive and carries the neural 
crest ventrally and slightly cephalad in the region of the olfactory 
and optic vesicles. The olfactory capsule is now completely 
separated from the brain wall by a sheet of ectodermal cells 
which, extending ventral and caudal from the olfactory capsule, 
reaches the ventral limit of the brain wall below the optic vesicle. 
