MUSEUM OF COMPARATIVE ZOOLOGY. 181 
in the paragraph especially devoted to the median eye (p. 531), the 
following occurs: “ Die Mittelaugen werden von der gleichen Falte* 
gebildet, welche am Baue der Kopflappen Antheil nimmt, nur mit dem 
Unterschiede, dass fiir den Bau des Hirns die tiefen Theile der Falte 
verwendet werden, wihrend die Augen Derivate der peripherischen Theile 
derselben Falte* sind.” After speaking of an accessory fold and a new 
fold in connections already alluded to, the statement that the cephalic 
lobe and median eye are derived from the same fold seems to me 
contradictory. 
The involution of the optic sacs in spiders, as Locy has shown (’86, 
Pl. XI. fig. 70), takes place at a much later period than the formation of 
the cephalic ganglia, and to all appearances independently of the latter. 
Whether in scorpions the growth of these two structures is connected or 
not, is a question for the determination of which I have not yet secured 
the requisite material. Besides the statements of Kowalevsky and Schul- 
gin, which are somewhat obscure, there remains as a guide only the 
analogous case in spiders.; the fact that the later stages in the eyes of 
spiders are essentially the same as those of the scorpion, lends support to 
the view that the eyes in scorpions, as in spiders, arise from folds inde- 
pendently of those concerned in the formation of the brain. 
The most noticeable changes which the pair of sacs undergoes in the 
further development of the eyes are, first, an obliteration of their cavities, 
and, second, a considerable change in position. The closure of the sacs 
is first effected in the region where they unite with the common neck. 
From this point the fusion of the retinal and post-retinal layers proceeds 
toward the blind end of each sac, and the neck, becoming detached from 
the sacs, is slowly withdrawn to form a part of the permanent hypodermis 
(Pl. IL. fig. 11, col.). The line of demarcation between the retina and 
post-retina at the deep end of the sac is the last trace of the already ob- 
literated cavities (Pl. II. fig. 10, cav.). 
The change in position undergone by the eyes is correlated with a 
change in the form of the animal’s body. In the embryo the region of 
the prosomatic shield occupies the anterior face of the animal, and there- 
fore lies in a plane approximately perpendicular to the long axis. The 
optic sacs are situated near the centre of this region, the plane of their 
flattening being nearly vertical, and the lines corresponding to the future 
axes of the eyes being horizontal. As the animal develops, the shield 
assumes a position more nearly horizontal, till at length it becomes en- 
tirely so. The axes of the eyes, having shifted through an arc of 90°, 
* The Italics are not in the original. 
