28 THE AMEKICAN MONTHLY [February, 



It is not obviously cellular in character but banded with alternate stripes 

 of material denser or more open. It is not a tissue of cells, but a con- 

 densed secretion poured out from the cells of the layer next below and 

 hardened by contact with the air or water. 



2. The cornea, which is directly continuous with the cuticle of the 

 stalk, is plainly seen at the point of junction of the two to be of the 

 same composition, viz., layers of non-cellular matter, but it is unlike 

 the cuticle in two respects, — it is thinner and more compact, not being 

 made of alternate denser and more open layers, and it is seen in the 

 section to be divided up into blocks (see fig. 6, f ) , which fall oppo- 

 site the retinal elements. These blocks or facets can be seen best in a 

 surface view, which may be made with the low power without any 

 special preparation. They are thus seen as four-sided areas into which 

 the entire corneal cuticle is subdivided. Each is further seen to be 

 slightly convex outward. In life it acts as a lens, and this peculiarity 

 of the eye, so common in all the crustacians and insects, has given origin 

 to the name " compound eye," by which such eyes are commonly desig- 

 nated. If desired, a small portion of the cornea of an eye can be sliced 

 off parallel with the surface, the inside removed by short maceration in 

 5% potash solution and mounted in glycerine jelly. It will show these 

 corneal facets as a very beautifully regular tessellated pavement. A 

 comparison of the cornea will show that both its peculiarities cease at 

 the basilar membrane, no facets being present over the stalk and the 

 cuticle being there less uniform and dense, but thicker. Such a modi- 

 fication of the skin, to serve a particular purpose instead of the intro- 

 duction of a new sort of substance for a new purpose, is very character- 

 istic of the mode of building found in organisms. The transparent 

 cornea of our own eyes is no wise different in general character from the 

 skin of our faces, though the one is utterly opaque and the other very 

 transparent. 



3. The hypodermis in the living eye and over the entire body as well 

 is a thin tissue of living cells from which the outer cuticle is produced 

 as an excretion. The hypodermis forms a sort of cone, upon which 

 the outer skin is formed as an entirely lifeless product. In the stalk the 

 cellular character of the hypodermis can be very readily seen from the 

 number and position of the nuclei as well as from the position of occa- 

 sional cell-walls to be an ordinary columnar epithelium, In the retinal 

 chamber of the eye, however, the character of the hypodermis is not 

 at all plainly cellular (in any of my sections), but it can be seen as a 

 structureless thin strip (see fig. 6 between f and c and c, also fig. 1 c 

 h), which usually tears away from the cuticle and follows the retinal 

 rod or crystalline ctme, though usually quite distinct from it. It is 

 really cellular as well as that of the stalk. Within the hypodermis the 

 eye structures fall into two distinct sorts — those of the anterior cham- 

 ber of the eye or the retina, and those of the stalk. Let us study first 

 the section of the retina. 



4. The Visual Rod (taking the name used by Professor Huxley to 

 designate the entire structure shown in figure 4 of the plate) is a trans- 

 parent body consisting of three different, unlike parts. Outwardly, 

 near the hypodermis, it is broadest, and from this it tapers, at first 

 slowly then more rapidly, as it runs inward. At its inner end it en- 

 larges to a spindle-shaped swelling, which contracts again as it reaches 



