100 PATTEN. [Vou. II. 
A large nucleus appears in the central area, which in- 
creases in size as the latter becomes more and more elongated. 
It finally lies in about the middle of the dark area, between the 
two rows of nuclei (Fig. 6a). 
The explanation of the surface views will be found on ex- 
amining Pl. XI., Figs. 61-63, which represent sections through 
the eye while in the stages shown in Figs. 2, 4, and 6. 
It will be seen that the median, dark area corresponds to a 
broad elevation of the ectoderm, in which the nuclei are 
crowded toward the surface, the superficial ones being slightly 
stained by my method of preparing surface views. On either 
side of the ridge the nuclei are deeply situated and unstained, 
hence the clear area surrounding the ridge. In order to save 
time and expense I have, in most instances, indicated the surface 
nuclei by shading. The elevation gradually narrows and sinks 
to the level of the surrounding ectoderm, or even below it, until 
it is reduced to a narrow ridge with a shallow furrow along its 
summit. On either side of this furrow a single layer of nuclei 
extends to the surface of the ridge (Fig. 63). The most super- 
ficial of these nuclei are the ones which appear as a double row 
in surface views. 
The clear band surrounding the dark area soon loses its regu- 
lar outline and becomes distinctly four-lobed (Figs. 6 and 6a), 
each lobe being a depression in the ectoderm, beneath which 
the nuclei are arranged much like those in the whole ocellus at 
a later period (compare Figs. 62 and 64), or like those found in 
separate sense organs. Each depression represents, in fact, a 
distinct sense organ, or eye. They form the greater part of 
the retina of the future ocellus. The two posterior pits are 
deeper and larger than the anterior ones, hence the whole 
clear area is somewhat conical. 
There is a dark spot, with a clear area surrounding it, on 
the anterior dorsal edge, and one on the median ventral edge 
of the clear area (Fig. 6a, 5 and 6). These clear spots resem- 
ble the main clear area, with which they are continuous, in 
structure and general appearance. 
All these clear areas and dark spots form one thickened patch 
of ectoderm, which soon becomes more regularly oval, and the 
pits and the dark streak disappear. The whole organ is being 
invaginated to form the floor of the deep depression seen in 
