MORPHOLOGICAL STUDIES. 
63 
such a pigmentless retina of Petromyzon under very high 
power. 
Fig. 7 is taken from exceedingly good sections of such 
parietal eye, and is drawn under Zeiss’s objective F. 
The end elements are shown to be of two kinds, comparable 
as it seems to me to those in the retina of the ordinary eyes. 
By far the most numerous are the long rods ( rd .), but in 
addition and between the latter one finds a few cones ( cn .). 
The mode of connection of these end elements with the 
“nuclear layer” is also figured, as well as their termination 
in an outer ganglion cell layer {rftgl.). 
The Parietal Eye in Myxine. 
Although I have examined many Myxine in a better or 
worse state of preservation, I have only found one in which 
the structure of the organ in question could be well made out. 
In fig. 12 I have drawn the general appearance and relation- 
ships of the organ as seen in sagittal section under low magni- 
fication. The eye is a large flattened organ lying within the 
skull and connected to the thalemencephalon by a short, thick 
solid stalk (fig. 12, st.). In the specimen under description 
it contained no pigment, the dark portion of the vesicle 
in fig. 12 being only the optical appearance of deeply stained 
nuclei in a thick section. The epiphysis is here undivided 
into two vesicles, in which respect it differs from the corre- 
sponding organ in Petromyzon. Both anterior and posterior 
walls have practically the same structure, although the 
posterior wall shows the elements in a slightly better developed 
condition. A piece of the retina (posterior wall) under high 
magnification is figured in fig. 11. It shows that the struc- 
ture is made up of a row of rod-like nucleated cells which 
taper towards their bases. The tapering bases probably end 
in some of the cells which form a scanty outer layer to the 
retina. The nuclei of the rod-cells lie not very far away from 
the cavity of the vesicle. Through the vesicle a number of 
longitudinal fibres or striae pass (fig. 11). One may compare 
the retina here described to that of Cyclodus as figured by 
