178 BULLETIN OF THE 



scorpions were three-layered, and that they were probably formed from 

 a cup-like involution of ectoderm. The closure of the cup produced an 

 optic vesicle, the deeper half of which became retina, while the more 

 superficial half was probably represented by a structure to be described 

 hereafter, the preretinal membrane. The details of this method of de- 

 velopment, as will be seen later, are not confirmed by my observations ; 

 but nevertheless it remains to Patten's credit that he was the first to 

 insist that the eyes in scorpions were three-layered, and not two-layered 

 as had been previously held. 



Metschnikoff ('71, p. 225), in his paper on the development of the scor- 

 pion, did not discuss the formation of the eye further than to claim for 

 it a hypodermal origin. His evidence on this point can scarcely be consid- 

 ered as conclusive, for his studies were made from superficial views only. 



The youngest material at my disposal was already somewhat advanced; 

 but the eyes were still sufficiently undifferentiated to give adequate evi- 

 dence as to their origin, and thus to afford a trustworthy basis for the 

 interpretation of structures in the adult. 



In the earliest stage examined, the eyes appear on surface view as a 

 pair of oval, slightly pigmented areas. They are situated at the anterior 

 end of the head, one on either side of the median line, and somewhat 

 above the mouth. In a slightly older stage a sagittal section a little at 

 one side of the median plane shows the region of the pigmented areas to 

 be already composed of three layers of hypodermis (PI. III. fig. 12, prr., 

 r., and j) r.). The hypodermis of the prosomatic shield {pr r.) extends 

 downward toward the mouth, and preserves its indifferent condition ; 

 before reaching that opening, it is folded upon itself, the deeper arm (r.) 

 of the fold passing dorsally in contact with, the deep face of the external 

 portion. The ventral third of the infolded layer is as thin as the ex- 

 ternal layer of hypodermis, but the remaining two thirds are considerably 

 thickened and contain much pigment. This thickened layer, becoming 

 rapidly thinner at its dorsal end, is also folded upon itself to form a 

 third layer (/> r.), which passes ventrally next the deep face of the thick- 

 ened portion, and at the point of first folding becomes continuous with 

 the external hypodermis as it proceeds in the direction of the mouth. 



This condition is practically an involution of the hypodermis. The 

 infolded layers take the form of a flattened sac, or pocket, the open end 

 of which is situated in the median plane between the mouth and the 

 previously described pigmented areas. From its opening the pocket 

 extends vertically upward, and its anterior face is closely applied to the 

 deep surface of the permanent hypodermis. 



