170 PATTEN. [Vot. II. 
In the centre of each group of four sensory pits, is a single large nucleus 
of doubtful significance. 
(4) The pits of each eye finally unite to form a thickened patch of 
ectoderm, with a median double row of gigantic cells and a common 
cuticular thickening. 
(5) The thickened ectoderm is invaginated to form an optic vesicle, 
the inner walls of which form the retina, while the surrounding indiffer- 
ent ectoderm forms a third layer of cells over each vesicle, thus produc- 
ing a typical three-layered eye. 
(6) In the embryonic stages of eyes I.-IV., the retinas of which are 
invaginated without the formation of a cavity in the optic vesicle (unless 
the space between the median row of gigantic cells can be called one), 
all the rods are horizontal. 
(7) In the full-grown larvz, the smaller outermost rods become up- 
right ; the larger and deeper ones remain in a horizontal position. 
(8) In eye V., there is at first a strong tendency to form horizontal 
rods. But the laterally flattened optic vesicle expands, forming a 
spacious cavity in the vesicle, and all the rods become upright except 
those of the median row of gigantic cells. 
(9) In eye VI., which has no median row of giant cells, no horizontal 
rods are formed. 
(10) The outer wall of the optic vesicle in eyes I-IV. seems to be 
absent. In the embryos, its presence is indicated only by a few char- 
acteristic nuclei between the retina and the corneagen. 
(11) In eye V., the outer wall of the optic vesicle is represented by 
two great masses of inverted, rod-bearing cells, probably derived from 
the two sensory spots, 5 and 6, seen in surface views of the eye before 
invagination. 
(12) In eye VI., the outer wall is composed of a thin nucleated 
membrane, and a cluster of inverted retinal cells, derived from sense 
organ number 6. 
(13) Eye I. is composed of at least nine sensory spots, four of which 
with their central nucleus and median row of giant cells give rise to the 
horizontal retina ; four more, exactly like the first, to the vertical retina ; 
and the ninth, to the appendage. All these sense spots unite to form a 
single homogeneous organ ; but, during the later stages, the three groups 
of sensory spots become greatly modified, so that in the adult eye the 
parts they give rise to, the vertical, and horizontal retinas, and the ap- 
pendage, are widely different in structure. 
(14) All the retinas are composed of retinophore, formed by the 
union of two cells. They contain two nuclei and two rods, and are 
supplied with axial, and external nerve fibres. 
