118 PATTEN. [Vou. IL. 
on the surface; but during the succeeding stage, its anterior 
end is pushed into the tissues of the head, as though the whole 
plate swung on a pivot at the posterior end, through an angle of 
ninety degrees. The long axis of the retina then lies at right 
angles to the surface, and in a semi-transparent preparation, 
like that shown in Fig. 9, we look along the retinal furrow, not 
down into it as in Fig. 8. It is not easy to form a clear mental 
picture of the way in which the retinas of eyes I. and III. are 
invaginated. To make matters clearer, let us suppose the head 
to be made of soft india-rubber. Then if a slender but inflexi- 
ble rod be laid between the two rows of nuclei of the median 
ridge, as fast as it is pushed below the surface, the lips of the 
indentation that will tend to form, come together, conceal the 
rod, and prevent the formation of any cavity. If this process 
is continued until all the sensory cells on either side of the 
rod are invaginated, a long narrow pocket will be formed, com- 
posed of two thick walls, whose originally outer surfaces touch 
each other, bringing their cells end to end and at right angles 
to the opening of the pit (Fig. 85). There are no transitional 
cells connecting the inner edges of the two walls, so that, strictly 
speaking, there is no floor to the pocket. This completes the 
first stage of the invagination. Now, in eye III., suppose the 
posterior end of the rod is fixed and the opposite extremity 
swung inwards through an arc of ninety degrees; the retina 
will then be brought with its long axis at right angles to the 
surface, and the second stage will be finished. Finally carry 
the rod in this position backwards a distance equal to the height 
of the future corneagen, and, without considering minor details, 
the third and last stage is completed. 
In eye I. the pocket has a bend like that of the dark area 
(Figs. 14-16) which divides the retina into two unequal parts. 
Between the lateral walls of the posterior portion, or that which 
gives rise to the horizontal retina, is situated the first large 
nucleus (zc’., Figs. 17 and 85), and in a corresponding. position 
in the anterior part, which gives rise to the vertical retina, is the 
second large nucleus, #c.2 The whole pocket is of nearly uni- 
form depth, as shown in Fig. 17, which represents a digram- 
matic outline of the whole eye seen as a transparent object. In 
order to continue the invagination by means of our rod, it must 
be cut at a point a little back of the second nucleus, zc.%, and 
