MUSEUM OF COMPARATIVE 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- 
rating 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 fully 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, nl. 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 eyes have been 
examined has a nucleus remained in a prebacillar position. 
VOL. XIII. — No. 6. 13 
