192 BULLETIN OF THE 



As to their nature two suggestions have been maae. juankester and 

 Bourne ('83, pp. 185, 186) imply that they are of the nature of rhab- 

 domes; in this light they are further discussed by Mark ('87, p. 93). 

 Patten ('86, p. 684) is inclined to look upon them as degenerate nuclei. 

 In Centrurus the phaospheres, being of nearly the same size as the nuclei, 

 present less favorable opportunities for study than in those scorpions 

 where they are much larger. Those in Centrurus stain in much the 

 same way that the surrounding nuclei do, and in fact are to be distin- 

 guished from these mainly through their highly refractive dots. In many 

 cases, however, these dots are not well marked, and it is then difficult 

 to determine whether a given body is a nucleus or a phaosphere. The 

 nuclei are constantly oval in form; the phaospheres are more or less 

 irregular in outline. This irregularity, however, is only noted in phao- 

 spheres which have very refractive dots, and never in those which seem 

 to be transitional in form between nucleus and phaosphere. 



The third type of cell occurs as a single layer of pavement-like ele- 

 ments at the back of the retina (PI. II. fig. 4). It has been correctly 

 stated by Graber ('79, p. 84) that this pavement layer is the matrix of 

 the sclera. Lankester and Bourne ('83, p. 192, pi. X. fig. 8, />) have 

 also observed it in Androctonus, where the cells are relatively much 

 smaller than in Centrurus. In sections of Centrurus the outlines of 

 these cells are visible, though faint ; in form they are broadly columnar. 

 Their nuclei, as previously stated, take a deep color, are flattened, and 

 are always located at the end of the cell farthest from the sclera. 



This deep layer of cells envelops the convex face of the retina, passing 

 up on its sides till it reaches the edge of the retinal cup (Lankester and 

 Bourne, '83, pi. X. fig. 8, p). Here, as Graber has shown ('79, PI. V. 

 fig. 14), it becomes continuous with the retinal layer. Only in the re- 

 gion where the retinas of the two median eyes fuse does this basement 

 layer fail to cover the deep surface of the retina proper. 



The principal histological changes which take place during the devel- 

 opment of the eye relate to nuclei, the pigment, and the optic nerve. 

 The formation of the optic sacs, the disappearance of their common neck, 

 and the fusion of the post- retinal with the retinal layer has already been 

 described. 



While tiie eye is yet an ectodermic pocket (PI. III. figs. 12-15), the 

 nuclei are distributed through the whole of the thickened retinal layer ; 

 in the post-retina they form a single row. At this stage the nuclei of 

 the different cells are indistinguishable. Their outlines are round or 



