214 BULLETIN OF THE 



(rd.")., while those in zone 5 are much elongated in the radii of the ves- 

 icle (nl.J"). The suggestion at once comes that this difference is due 

 solely to the crowding togetlier of the cells nearest the internal surface 

 of the retina, and hence that the two zones should in reality be re- 

 garded as but one. If, howiwer, the difference in shape of the nuclei 

 were the result solely of such crowding, we should find a complete 

 gradation from the spherical to the elongated form in passing from 

 without inward ; but such a gradation is not found in fact. Further- 

 more, on close examination with high powers, it is found that the nuclei 

 differ in structure as well as in form. An irregular stellated area can 

 be detected in the centres of some of the spherical ones which does not 

 exist in the elongated ones ; also, the entire substance of the former 

 is slightly more granular than that of the latter. In the fifth zone 

 cell boundaries (though not well shown in the figure) can be quite dis- 

 tinctly traced to the internal basement membrane : but how the cells 

 of the fourth and fifth zones are related I have been unal)le to deter- 

 mine, since cell boundaries in the fourth zone cannot be traced. (G) The 

 last layer may be designated as an internal basement membrane {mb. 

 ba. i.), though it differs somewhat ii) structure from the external base- 

 ment membrane, being of a granular nature. It extends over the 

 surface of the lens, as well as over the retina, and is rather more com- 

 pact in the former than in the latter region. Projecting into the cavity 

 of the vesicle from the retinal portion are found certain structures con- 

 cerning the nature of which I am not quite sure, but believe them to 

 be secretions from the cells of the fifth zone. They are in general 

 elongated, and pointed at their free ends, though their outlines are 

 ragged and indefinite. They always stain most deeply at their internal 

 free ends. In many ca.ses, as at *, they are seen to be continuous with 

 the cells of the fifth zone through the internal basement membrane. 

 These structures may correspond to v/hat de Graaf has described and 

 figured as existing on the internal surface of the retina of Anguis, and 

 has called " Staafjeslaag," but which Spencer and otliers believe to 

 be merely a coagulum from the fluid that probably filled the cavity in 

 the recent state. It is, however, scarcely possible to account for the 

 structures here under consideration in this way, as is to be seen from 

 my description and figures of them ; furthermore, a coagulum {cog^ 

 does exist in addition to these. 



Within the substance of the retina (Fig. 5, va. rfn.) are found a num- 

 ber of cavities varying in diameter, as measured in the plane of the 

 sections, from 5.5 /a to 22 /x. The sections of these cavities are never 



