58 BULLETIN OF THE 



nected with the author's theory of an intrusive (mesoblastic) connective 

 tissue. 



At least three possibihties 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 in situ ; (2) they may be cells which have been 

 detached from the posterior layer of a retinal involution, and have grown 

 in between the retintd^e 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 in situ ; 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 lo 

 this question by other means tlian 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 di^Terent 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 otlier 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 jilace and the direction of the invagina- 

 tion. (See theoretical considerati(ms, below, pp. 91, 92.) 



Perhaps Metschnikoff ('71, p. 22.'J, 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. 



