76 BULLETIN OF THE 
of these figures, however (Fig. 9), there is some evidence that the row of 
nuclei (¢ap.) is not single, but double, and that it is the result of an out- 
folding of cells (tap.) lying between the regions pr. and 7. This conelu- 
sion is strengthened by the condition of the anterzor lateral eye as shown 
in Fig. 4, tap. It is almost certain, from the shape and direction of the 
nuclei, that the equivalent region in this case is a fold, open below. If 
this middle region really represents a double rather than a single layer 
of hypodermal cells, then the S-shaped appearance is deceptive ; and one 
must suppose that half of the fold has become merged in one of the 
thickenings (or otherwise obscured), while the other half remains as the 
only apparent means of connection between the two thickenings. It is 
further evident that this owtfolded middle region must be in the nature 
of a reentrant fold from the apex * of an original zvolution, of which 
the two thickenings constitute the walls. 
The condition and connections of this middle region are of great im- 
portance in deciding upon the morphological relations of the retina, and 
it is therefore to be regretted that the evidence as to its real nature is not 
more conclusive. 
In the tract nearest the anterior median eye (Fig. 8, p 7.) the thickening 
results simply from a displacement and a slight elongation of the cells 
and their nuclei, the latter overlapping each other like so many tiles. But 
the posterior thickening is more complicated ; it consists of two parts. 
The anterior part is composed of cells, the nuclei of which have their 
long axes nearly parallel with the surface of the head; they collectively 
form a broad band (7.) nearly perpendicular to the surface of the head ; 
the nuclei are wedged between each other so as to form two or three 
irregular rows. Behind this, and more or less in continuity with it, is a 
region (pr 7.) which gradually diminishes from a thickness nearly equal- 
ling the length of the ‘‘ perpendicular band,” to the thickness of the or- 
dinary hypodermis. The nuclei in this triangular region are, in the main, 
perpendicular to the surface of the head, although showing a tendency to 
radiate from a point near the deep end of the “band.” There are, then, 
four more or less distinct tracts already recognizable. These may be 
named from behind forward, pre-retinal (pr r.), retinal (r.), tapetal (tap.), 
and post-retinal (p r.), respectively. The same regions may also readily 
* It is possible that the re-entrant fold was not confined to the bottom (apex) 
of the eye-pocket, but extended along its margins, and that the ‘‘ fissure” in the 
tapetum, subsequently referred to, is to be explained as resulting from the failure of 
these two lateral ingrowths into the pocket to unite along the axis of the latter. 
