MUSEUM OF COMPARATIVE ZOOLOGY. 181 



in the paragraph especially devoted to the median eye (p. 531), the 

 following occurs : " Die Mittelaugeu werden von det' gleichen Falte * 

 gebildet, welche am Baue der Kopflappen Antheil nimmt, nur mit dem 

 Unterschiede, dass fiir den Bau des Hirns die tiofen Theile der Falte 

 verwendet werden, wahrend 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 t.he optic sacs in spiders, as Locy has shown ('86, 

 PI. .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 tlie 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 the sacs, is slowly withdrawn to form a part of the permanent hypodermis 

 (PI. II. 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 (PI. 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 



