58 BULLETIN OF THE 
nected with the author’s theory of an intrusive (mesoblastic) connective 
tissue. 
At least three possibilities may be suggested to explain the inter-reti- 
nular pigment-cells discovered by Lankester and Bourne in the central 
eyes of scorpions: (1) They may be developed from indifferent hypo- 
dermal cells practically 2m setu ; (2) they may be cells which have been 
detached from the posterior layer of a retinal involution, and have grown 
in between the retinule from behind; or (3) they may be, as claimed 
by the authors, intrusive mesodermal cells. 
If the lateral eyes are really “ monostichous,” that would seem to afford 
an argument in favor of the first possibility, the interneural cells of the 
lateral eyes being really pigment-cells developed im stu ; and in that 
case the “ inter-retinular pigment-cells ” of the central eyes would cor- 
respond to the interneural cells of the lateral eyes. 
The above-quoted arguments (pp. 56, 57) in favor of the third possi- 
bility do not seem to me to outweigh the fact that it is the hypodermis 
and its derivatives which have in Arthropods the greatest tendency to 
the pigmented condition. 
Finally, the intimate connection between the other pigmented cells 
and the “intracapsular epithelium” would be favorable to the second 
view, —at least I cannot regard the intrusion between the retinal ele- 
ments of pigment-cells from this source (posterior layer of the involution) 
as any less probable than their migration through the “ ommateal cap- 
sule ” and the intracapsular epithelium.* 
No one, however, will think of arriving at a conclusive answer to 
this. question by other means than a careful histogenetic study of the 
developing eyes of some of the scorpions. 
So far, then, as regards the median (central) eyes of scorpions, they do 
not present conditions sufficiently different from those of spiders to pre- 
vent a similar interpretation of their parts. With the lateral eyes, how- 
ever, the case is quite different. If the recent researches of Lankester 
* There are other indications, besides that of a triplostichous condition, which 
point to the probability of an involution of hypodermis as a source for all the post- 
vitreous portions of the ommateum. In the scorpions, as well as in the spiders, the 
emergence of the optic-nerve fibres is so eccentric (especially in Androctonus) that 
one might almost venture to predict even the place and the direction of the invagina- 
tion. (See theoretical considerations, below, pp. 91, 92.) 
Perhaps Metschnikoff (71, p. 225, Taf. 16, Figg. 10, 11) was very near to discover- 
ing the true relation of the eyes to the hypodermis when he explained that they 
appeared as thickenings of the dermal fold which forms an overgrowth over the 
cephalic ganglia. 
