8 BULLETIN OF THE 



face is hollowed, however, so that its centre is the thinnest part. In a 

 few cases the corneal hypodermis has appeared as cubical blocks, rather 

 than as tiles. This thickened condition probably indicates an increased 

 functional activity, and the more frequently occurring tile-like condition 

 may correspond to a quiescent stage. 



The two nuclei contained in each square are placed some distance 

 apart, and on one of the diagonals of the square (Fig. 3). Their long 

 axes are approximately parallel to the other diagonal. In a given eye all 

 the squares agree in having the nuclei on parallel diagonals. The pres- 

 ence of two nuclei in a square indicates that the square consists of two 

 cells. Any membrane separating the two cells must necessarily pass 

 between the two nuclei, but all attempts to discover such a membrane 

 have failed. However, for a reason which will be given shortly, I be- 

 lieve that the protoplasm of the hypodermal square is divided by the 

 diagonal w r hich lies between the nuclei. In the centre of each square 

 several oval or round outlines are usually visible (Fig. 3). These are 

 vesicular bodies which occur in the distal ends of the cone-cells, and 

 which can be seen through the very thin corneal hypodermis. 



The corneal cuticula is the result of the activity of the corneal hypo- 

 dermis. Viewed from the surface, the cuticula is divided by narrow- 

 bands into square facets (Fig. 2). Each facet is external to a hypoder- 

 mal square. The proximal and distal faces of each facet, as can be seen 

 in the transverse section (Fig. 1), are very nearly flat, the proximal 

 face only being a trifle convex. This convexity, however, is so slight 

 that one cannot attribute to the facet the character of a lens. 



When a piece of corneal cuticula is cleaned by treating it with potas- 

 sic hydrate, and is then examined in water, the markings which are 

 visible with difficulty in preparations mounted in balsam are easily seen. 

 Each facet in addition to its narrow limiting bands has a faintly marked 

 diagonal band which divides the square into two equal triangles (Fig. 2). 

 In the different facets of a given eye the diagonal bands are parallel. 

 Newton (73, p. 327, Plate XVI. Fig. 3), in describing the structure of 

 the eye in the lobster, states that each facet is crossed by two diagonals 

 at right angles to each other. This statement I cannot confirm, for, 

 although I have searched with care, I have never succeeded in finding 

 more than a single diagonal in each facet. In the middle of the diagonal 

 there is an irregular hazy patch. This at times has a distinctly marked 

 cross in it. When the cross is present, one of its axes lies in the diago- 

 nal band, the other extends at right angles to the band (Fig. 2). 



Whether all of these markings extend through the substance of the 



