78 BULLETIN OF THE 
slightly S-shaped. The line of demarcation between this and the second 
tract, or next deeper layer, is not always sufficiently distinct to allow one 
to claim with certainty the presence of an internal cuticula (basement- 
membrane) equivalent to the pre-retinal membrane of Graber. In some 
cases (Fig. 24) I have seen a sharp limiting membrane between the pre- 
retinal and retinal layers; but in other cases (Figs. 20-22) it has been 
impossible to find the least indication of such a membrane. The form 
and relation of these two tracts indicate a gradual slipping of the first 
upon the second, rather than a typical folding ; but this is probably to 
be considered as simply a modification of what originally was a true fold- 
ing at the retino-lentigen margin of the retinal pocket. The overgrowth 
of the lentigenous cells finally results in the same relation between the 
two tracts as was originally produced by the zngrowth (infolding) of the 
retinal layer. In the original method the retinal layer formed one wall 
of a free pocket (compare Locy, ’86, Pl. X, Fig. 64); in the modified 
process it is from the beginning in contact with the lentigen. The pos- 
terior region of the latter is finally extended (Figs. 11, 12) so as partially 
to envelop the posterior margin of the retina. 
The relations of the second tract (r.) are not equally clear upon all the 
sections. If Figs. 8 and 9 were taken to represent the original unmodi- 
fied condition of the hypodermal foldings, the conclusion might be that 
there had been an outfolding having the second tract for its wall on one 
side, and the third tract on the other. If this were the typical method, 
there could be no doubt but that that face of the second layer which at 
this stage is directed forwards, and in which are developed the bacilli, 
would correspond to the originally deep surface of the hypodermis. The 
bacilli would therefore be developed here, as in the anterior median eyes, 
at that end of the cells which in the original position of the hypodermis 
must have been turned away from the light. But of the justice of this 
conclusion I am not convinced ; for in other cases (Fig. 4) the out- 
folding, as stated above, appears to involve only the second tract, and in 
still others there is not sufficient evidence of a true folding of any kind. 
In Figs. 2 and 16, for example, the conditions are such as might have 
been produced by a detachment (delamination*) of the cells of the third 
tract (tap.) from those of one of the adjacent layers, without the forma- 
tion of any outfolding. If either of the latter suppositions represents 
the true state of the case, then the anterior face of the retinal tract (7.) 
* The delamination might possibly have resulted from an abbreviation in the 
process of forming the tapetum, which originally took place exclusively by means 
of an outfolding of the tapetal cells. 
