A. R, Galloway 
271 
Fig. IV. (x Zeiss D. logwood stain.) 
Section through iris, showing the anterior pigment spot seen in Fig. I. 
The spot is seen to be due to the presence of dark brown pigment cells of 
a similar nature to the stroma pigment cells, several of which, whole or dis- 
integrated, appear in the field : the logwood staining shows the structure of the 
iris tissues. 
Professor Dean suggests that these stroma pigment cells are possibly connective 
tissue cells that have the power of movement. These sections appear to show that 
they tend to pass from the pigment epithelium layer forward through the stroma, 
to distribute pigment either in the shape of pigment cells or granules to the 
anterior surface of the iris. Whether this view be correct or not the sections 
lend no corroboration to the idea that there are two kinds of pigment, a blue-black 
behind, and a yellow-brown in front. All the appearances are compatible with 
the belief that only one pigment is present, which when heaped up in the form 
of pigment cells, is dark brown in colour, and is yellow when thinly distributed 
especially in a granular form. Moreover the dark brown pigment cells of the 
anterior spot are exactly similar in colour to the retinal pigment epithelium of 
the posterior surface. 
It appears probable from the presence of yellow anterior granular pigment in 
all sections of the median and peripheral zones which were blue and apparently 
free from pigment, that many so-called pure blue eyes may have this anterior 
coating of granular pigment, which is revealed only microscopically*. Hence, in 
addition to the errors concerning two kinds of pigment, etc., another error in the 
classification of some blue eyes as having no anterior pigment would arise. 
Therefore any theory, Mendelian or otherwise, which depends on the sup- 
position that there are two kinds of pigments, with different loci, and that there 
is no stroma in the human iris, seems based on inadequate examination of the 
human eye. 
Hurst's results so far as they go give approximately the anticipated Mendelian 
proportions, but as I have endeavoured to indicate in this paper the amount of 
anterior pigment is a continuous quantity, and nothing short of microscopic post- 
mortem examination will determine to what extent an eye does or does not 
possess it. Hurst suggests that half the "clear" or pure blue eyes classified by 
ordinary observers really possess anterior pigment and that this is the reason why 
the data collected by Galton do not support the Mendelian theory. If, as I 
anticipate, many of Hurst's "simplex" irides would show some anterior pigment 
on microscopic examination, it is evident that he has been singularly fortunate 
in discovering that the limit of his personal method of examination was that 
which coincides numerically with Mendelian theory. This coincidence is the more 
[* In the case of the eyes of " clinically complete albinos" both in man and in lower forms of 
life a post-mortem microscopic examination will frequently show some posterior pigmentation, and 
occasionally some slight anterior pigmentation as well. On p. 85 of his paper Hurst states that in 
the albino pigment is entirely absent from both surfaces of the iris. Editor.] 
