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 (Pl. III. fig. 12, pr r., 
r.,and pr.). The hypodermis of the prosomatic shield (pr 7.) extends 
downward toward the mouth, and preserves its indifferent condition ; 
before reaching that opening, it is folded upon itself, the deeper arm (7.) 
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 (p7.), 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, 
