200 BULLETIN OF THE 



The horizontal sections shown in Figs. 22-27 (PI. IV.) are arranged 

 to represent the characteristic features of the ocular area of the left side 

 of the head, as one would observe it in passing from a dorsal to a more 

 ventral position. Calling that of Fig. 22 the first section, they are the 

 1st, 3d, 6th, 13th, 16tli, and 21st sections in a series from a single 

 animal. Fig. 22 represents the hypodermis directly above the eyes and 

 at the edge of the ocular area. The extent of this area is indicated by 

 the thickened region. Two sections below this (Fig. 23) the ocular area 

 is more extended, and shows a single simple depression (No. 1). It will 

 be observed that the band of nuclei indicates a more marked depression 

 even than the outline of the hypodermis itself. This simple depression 

 in the hypodermis indicates the position of a lateral eye. The cells 

 which compose the wall of the cup are wedge-shaped ; their nuclei are 

 below the middle of the cells, and those cells which occupy the central 

 portion of tlie depression are so attenuated at their free ends as scarcely 

 to reach the surface. The basement membrane (mb.) closely invests the 

 deep face of this structure, as it does any ordinary hypodermal thicken- 

 ing. The sixth section, Fig. 24, exhibits a region in which the ocular 

 area is greatly thickened, but it shows no depressions, and the nuclei 

 extend very near to the surface. Fig. 25, seven sections deeper than 

 Fig. 24, presents four cup-shaped depressions (Nos. 2, 3, 4, 5), each essen- 

 tially like the depression previously described. The two central depres- 

 sions (Nos. 3, 4) are the largest ; next in size is the anterior one (No. 2), 

 and smallest of all is the posterior one (No. 5). As in the case of depres- 

 sion No. 1 (Fig. 23) the band of nuclei in the region of each depression 

 forms a much deeper cup than the outer surface of the hypodermis. Tlie 

 basement membrane {mb.), as in Fig. 23, invests only the deep surface of 

 each hypodermal cup. From this plane ventrally the hypodermis gradu- 

 ally becomes thinner, and at the extreme edge of the dorsal shield the 

 indifferent hypodermis is reached. (Compare Figs. 18,20.) 



The five depressions just described are early stages in the development 

 of the lateral eyes. In the adult Centrurus only four eyes are present. 

 Of the five depressions seen in the embryo the most posterior (No. 5) 

 of the ventral four disappears, and three remaining form the " princi- 

 pal " lateral eyes. The fourth or " accessory " eye arises from the dorsal 

 depression (No. 1), which, even in the embryo, occupies a position above 

 the space between the second and third depressions (Nos. 3 and 4) of 

 the lower row. The presence in the embryo of a rudimentary fifth eye 

 is interesting, in view of the fact tliat there are five eyes in the adult of 

 Androctonus, as has been shown by Lankester and Bourne. It is proba- 



