MUSEUM OF COMPARATIVE ZOOLOGY. 87 
area. By this process of infolding the whole region of the thickening 
becomes inverted, and finally lies under that portion of the still unmodi- 
fied hypodermis, which was at first just behind the thickening (Pl. X. 
figs. 63, 64), so that what was its external surface becomes its deep sur- 
face, and what was the deep surface lies relatively nearer the exterior. 
As a result of this infolding the region of the eye at this stage is com- 
posed of three distinct layers: an external (Figs. 64, 66, 4d), a middle 
(rén/.) and internal (rtv‘’.) layer. The external portion is composed of a 
single layer of cells, which at the outset do not differ from the hypoder- 
mis cells, with which they are continuous. The middle layer is the orig- 
inally thickened portion, and is composed of elongated cells, all having 
the same general inclination ; their nuclei are oval, being elongated in 
the direction of the long axis of the cells, and are arranged in three or 
four superimposed irregular rows. The internal layer, like the outer 
one, is composed of a single row of cells. 
The pocket of the invagination is not very broad, as is best to be seen 
on frontal sections (Pl. X. fig. 65). After a time the orifice of the inva- 
gination is closed by a fusion of its lips, and the retinal bulb, formed by 
the middle and internal layers, becomes separated from the hypodermis ; 
thus all direct evidence of its mode of origin is obliterated. 
After the invagination is completed the cells of the external layer 
begin to elongate ; they are now so closely crowded together that their 
nuclei are almost in contact, and the cell boundaries are not easily dis- 
tinguishable. Their nuclei also become lengthened, without losing much 
in thickness, until they are three or four times as long as broad. An 
accumulation of homogeneous faintly stainable substance appears be- 
tween the thin cuticula, which everywhere covers the hypodermis, and 
the free ends of these elongated cells; it is not at first sharply defined 
from the latter (Pl. X. fig. 66). This accumulation of substance finally 
causes an elevation of the surface, and just before hatching it has as- 
sumed a tolerably lenticular shape (Pl. X. fig. 68, dvs.). It gradually 
becomes more refractive, and a few days after hatching assumes nearly 
the form of the cuticular lens of the adult (Pl. X. fig. 69, dvs.). The 
lens is evidently produced by the secretive activity of the underlying 
elongated cells ; these have, in the meantime, continued to elongate, and 
their boundaries have become sharply defined; during this period the 
nuclei do not seem to share in the process of elongation, for after the 
formation of the lens they are seen to occupy the deep ends of the cells, 
and to be only a little larger than the nuclei of the adjacent hypodermis 
(Pl. X. fig. 69, hd). This layer of cells now constitutes the so-called 
