MUSEUM OF COMPAKATIVE ZOOLOGY. 193 



slightly oval ; their contents, except for a few sharply marked granules, 

 are very transparent. Somewhat later, but before the optic sacs have 

 closed, they are less abundant near the front face of the retina, but other- 

 wise no special arrangement is as yet evident. 



The rhabdomeres play an important part in the future distribution of 

 the nuclei. They first appear as light streaks, which, beginning close to 

 the preretinal membrane, gradually extend backward. With the exten- 

 sion of the rhabdomeres, the nuclei recede to the deeper parts of the 

 eye, and with very few exceptions * never occupy a place in front of the 

 rhabdomeres. 



At about the time the young scorpion is born,_ the cavity of the optic 

 sac having disappeared, the nuclei of the retinal layer are found to have 

 arranged themselves in two groups. In axial sections of the eye (PI. II. 

 fig. 9) one group forms an irregular line at the base of the rhabdomeres, 

 the other a broad band in the deeper part of the eye. The space sepa- 

 I'ating these two groups is considerable, and contains only a few scattered 

 nuclei. The deeper nuclei in the broad band, i. e. those nearer the sclera, 

 are to be referred to the post-retinal layer. 



At this stage the nuclei are still undifferentiated, and even after the 

 young scorpion has left the mother's back it is some time before one can 

 recognize differences between them. It is only in the fuUy developed 

 adult that a marked differentiation is reached. By this time the nuclei 

 of the retinal cells have become slightly more homogeneous (PI. II. fig. 

 4, 7il. r.) and somewhat reduced in size. The nuclei of the post-retinal 

 cells have become much flattened and stain more deeply. These, as 

 well as the nuclei of the pigment cells, are reduced in size, and have 

 become more homogeneous. The columnar " matrix " cells previously 

 described, and to which these flattened nuclei belong, constitute the 

 post-retina ; and their transition at the rim of the optic cup into the reti- 

 nal layer is only a preservation of the relation they have sustained to that 

 layer from the time of the original involution. This interpretation of 

 the "matrix" cells has already been maintained by Mark ('87, p. 56). 



The phaospheres appear at a very late date. In young scorpions 

 which have left the mother's back no trace of phaospheres was discover- 

 able, and it was only in those eyes in which the three forms of nuclei 

 were already distinguishable that the structures were noticed. The time 

 of their appearance — a period of nuclear differentiation — is evidence 

 in favor of their nuclear origin. 



* In only one instance out of the many in which developing eves have been 

 examined has a nucleus remained in a prebacillar position 

 VOL. xiii. — '■'o. 6. 13 



