56 T. H. MORGAN. 



Hi8tologt of the Eye. 



The following account of the histology is confined almost 

 entirely to a description of the middle and inner layer of the eye 

 of Phoxichilidinm. The shape of the cells of the outer layer is 

 sufficiently shown in Fig. 32. They are seen to be columnar, 

 and have conspicuous nuclei in the middle part of the cell. The 

 cells themselves show a bilateral arrangement, the smallest being 

 on each side of the middle line and thence towards the sides, 

 increase in length and breadth for a time, when they again 

 begin to get smaller, passing into the ectoderm of the body at 

 the point where the pigmented layer touches the surface. 



To obtain a knowledge of the middle and inner layers careful 

 macerations were necessary, and as the results obtained by the 

 different methods varied with the reagents used, it may be better 

 to mention those found most useful. The easiest method to 

 break up the eye into its constituent elements is maceration in a 

 modification of Haller's fluid. The following proportions were 

 used : Acetic acid (glacial) 5 parts, water 10 parts, glycerine 5 

 parts. The cupola bearing the eyes was cut off and quickly 

 immersed in the fluid, remaining there from a half to an hour, 

 washed in water, and teased out sometimes in water, sometimes 

 in glycerine. The method has one disadvantage in that it causes 

 the bacilli to swell up and become transparent, so that one may 

 be misled — as I was at first — by the apparent absence of bacilli 

 at the ends of the cells. The method has the great advantage in 

 that it separates the individual elements completely from one 

 another without breaking them up into pieces. Figs. 40, 41, 42, 

 44 and 45 were obtained in this way. A less successful reagent 

 was that of sulphuric acid and sea-water. It has the advantage 

 that the bacilli do not change their shape, but it does not easily 

 separate the cells. Osmic acid was not very successful, probably 

 on account of the difficulty of penetration. 



After the macerated pieces were put under a cover-slip a little 

 gentian violet was run under, which stained the cells and the 

 nuclei, rendering them easier to study. If one is successful the 

 middle layer breaks up into a great number of elements like 

 those shown in Figs. 40, 41 and 42. Each of these elements — 

 cells — is a somewhat flattened body with one clear end — the inner 

 — and a more protoplasmic middle portion containing a nucleus 



