MUSEUM OF COMPARATIVE ZOOLOGY. 37 



B and C is the greater extension of the membrane at stage C. (Com- 

 pare Figs. 39 and 41.) The retina is now much more completely 

 separated from the ganglion than formerly. The superficial and deep 

 portions of the proximal part of the disk are, however, still continuous. 



In stage D (Fig. 45) an important step is taken in the development 

 of the membrane. It splits at this stage into two layers, one of which 

 adheres to the retina, and the other to the optic ganglion. The retinal 

 and ganglionic layers of the membrane have probably been distinct from 

 the time of their formation ; but until stage D is reached, the double 

 nature of the membrane is not apparent. Occasionally in stage C one 

 notices at the point where the intercepting and basement membranes 

 unite a re-entrant angle (Fig. ±1, x). This in some cases extends a 

 short distance between retina and ganglion, and doubtless represents the 

 first step in the separation of the components which form the intercept- 

 ing membrane. 



In stage E (Fig. 46) the membrane has completely severed the super- 

 ficial from the deep ectoderm, and the separation of its retinal and gan- 

 glionic constituents extends over a broader area than in the previous 

 stage. 



The subsequent changes in the intercepting membrane consist in a 

 complete separation of its retinal and ganglionic portions. This separa- 

 tion is effected by the withdrawal of the optic ganglion from the super- 

 ficial ectoderm. In the adult, the ganglionic membrane remains relatively 

 thin, but the retinal membrane becomes much thickened. This mem- 

 brane, which has already been described as the basement membrane of 

 the adult retina, is not uniformly thickened, but presents local eleva- 

 tions, each of which is in the form of a cross. The four apertures which 

 pierce the membrane in the angles of the cross-shaped elevations, and 

 the relation which adjoining crosses and apertures bear to one another, 

 have already been described. The proximal face of the basement mem- 

 brane is nearly flat (Fig. 29). The cross-shaped elevations occur on its 

 distal face. The substance of the membrane is apparently homogeneous, 

 and contains no traces of cells or nuclei. The fact that its substance is 

 alike throughout favors the idea that it has been derived from a single 

 source. From stage E to the adult condition a few mesodermic cells 

 have been noticed next its proximal face (Fig. 1, cl. ms d.). These cells 

 are not intimately attached to it, and I am of opinion that they con- 

 tribute little or nothing to its composition. 



From the foregoing account I draw the following conclusions concern- 

 ing the growth of the basement membrane of the eye. In its earliest 



