40 MORGAN. [Vou. IX. 
The eyes on the apical plate are much more conspicuous 
than in the preceding stage. This is due to the greater 
amount of pigment developed, and to a more extensive pro- 
cess of invagination. A section of the apical plate is shown 
in Fig. 37. The plate is cut from right to left, so that both 
eyes appear. Between them is the central organ of the plate, 
but it is not so conspicuous as in the earlier stage. A deep 
pit leads on each side into the eye. The eyes are marked off 
from the surrounding ectoderm by the presence of a great 
amount of brown pigment, but the figure does not make the 
contrast between the eye and the surrounding tissue so great 
as in the actual sections. The outer ends of the pigment cells 
are clear, while the pigment is confined to an inner zone. The 
eyes bulge inwards, so that the punctuated layer is obliterated 
beneath them. The depth of the pit of the eye is greater than 
before, and a clear cuticle covers the outer ends of the cells 
bounding the pit. Sections in other planes show that the pits 
are continuous with the central cavity of the bulb. 
The tube leading to the mouth is shorter and its walls thicker 
than before, but the mouth lies some distance in from the sur- 
face. The oesophagus is strongly marked with pigment. 
Before joining the stomach it sends out a pair of short protru- 
sions from its antero-dorsal walls which represents the first pair 
of gill-pouches. 
The walls of the stomach are a little thicker than in the pre- 
ceding stage as seen in Fig. 38. Instead of a broad zone of 
ciliated cells on its ventral wall there is found now a hollow 
groove, and the few remaining cells with long cilia are now con- 
fined to the bottom of the groove. Whether or not all the 
cells of the earlier ciliated zone are turned into this groove I 
cannot say. The whole inner surface of the stomach seems to 
be covered with short cilia. 
The walls of the intestine are thrown into folds but the cells 
are not ciliated. A bunch of cilia projects into the stomach 
from the periphery of the hole between stomach and intestine. 
The anterior body-cavity is larger and its walls are thicker 
over the anterior portion of the structure. The proboscis 
vesicle is much as before. The second and third body-cavities 
