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 ingrowth (infolding) of the 

 retinal layer. In the original method the retinal layer formed one wall 

 of a free pocket (compare Locy, '86, PI. 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 ow^folding. If either of the latter suppositions represents 

 the true state of the case, then the anterior face of the retinal tract (r.) 



♦ 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 tutfolding of the tapetal cella. 



