180 BULLETIN OF THE 



the third or post-retinal layer. At the sides and blind end of the sac, 

 the two deeper layers, the retina and post-retina, are continuous. 



Granting the optic sacs to have arisen by involution, it is important to 

 notice that of the three layers described the retinal layer is inverted ; i. e. 

 that face which before involution is external becomes after involution 

 internal. A similar inversion in the retinas of the anterior median eyes 

 of Agelena has- already been demonstrated by Locy ('86, p. 87). In 

 fact, at this early stage, the only striking difference between the eyes in 

 Agelena and Centrurus is, that in the scorpion the two sacs are united 

 by a common neck, whereas in the spider they are independent involu- 

 tions. It seems scarcely possible that this is an essential difference, and 

 I therefore believe that the median eyes of scorpions, like the eyes of 

 spiders, arise from hypodermal involutions not immediately connected 

 with the formation of other organs. 



Since the above conclusion was arrived at, Kowalevsky and Schulgin 

 ('86, pp. 530-532), who have studied the development of Androctonus, 

 have published in a preliminary communication the results of their work. 

 In their description of the nervous system the development of the eye 

 occupies several paragraphs. On account of the absence of figures their 

 necessarily brief account is somewhat difficult to follow, and in one place 

 I am not sure of their meaning. 



Their statements on the median eyes are substantially as follows. A 

 pair of semicircular depressions occur in the cephalic plate. From the 

 anterior margin of each depression a fold grows down toward the mouth. 

 The closure of each depression by its fold gives rise to a right and left 

 cephalic vesicle. From the region at which the mouth of each vesicle 

 has just closed, a new fold develops. Each of the new folds opens 

 toward the animal's mouth, and takes on the form of a pocket. The right 

 and left pockets thus formed are the first traces of the median eyes. The 

 authors then describe the connection of the two pockets by a common 

 neck, and the thickening of the retinal layer, in the ventral part of which 

 pigment is deposited. 



As will be noticed, the description summarized has to do with the 

 earliest condition in the formation of the eye. The youngest stage of 

 my material is too advanced to permit me to make positive statements 

 on this subject. The point about which I have had difficulty relates to 

 the description of the brain and eye folds. In describing the formation 

 of the brain vesicles the authors speak of an accessory fold (eine acces- 

 sorische Falte, p. 530). When the development of the median eyes is 

 described, they speak of a new fold (eine neue Falte, p. 531). Finally, 



