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surrounding the developing eye have been increasing in length 

 during this process, the disc is actually less distinct than in 

 the larva in its first instar (fig. 78). So marked has been the 

 disparity in growth between these two parts of the ectoderm 

 that in the large larva, before the optic disc cells begin to 

 grow in size, the disc has undergone a distinct invagination 

 by the partial growth over it of the unmodified head ectoderm. 

 From now on, however, the disc gradually thickens, while the 

 ordinary ectoderm of the head becomes rather thinner. In 

 fig. 77, which is from a larva about sixteen hours before 

 pupation, we see the disc already sharply marked off from 

 the rest of the ectoderm. In fig. 79, which is taken from a 

 pupa thirty-six hours of age, the cells have elongated greatly, 

 and are beginning to turn inwards, towards the optic nerve, 

 as it grows out from the brain. Fig. 80 shoves a section of 

 the eye of a pupa which is about ready to emerge. The cells 

 have increased greatly in length, and the bases of the 

 ommatidia converge upon the optic nerve. Meanwhile there 

 has been a gradual increase in the convexity of the eye. (In 

 fig. 78, which is taken from an advanced larva, the eye is 

 shown as very much more convex than in the next stage, 

 taken some ten hours later. There is, however, no real com- 

 parison between the two, since the first is from a still actively 

 moving larva, in which the flexible disc must necessarily 

 be subject to considerable distortion.) 



The change in direction of the ommatidia probably finds 

 its explanation in the following observation. Beneath the 

 optic disc lies a series of tissues : the mesoderm of the body 

 wall (somatopleure), a membranous ingrowth of ectoderm, as 

 will be described later, and an outgrowth from the brain. (2) 

 These three form a fairly thick mass beneath the optic disc, 

 which later becomes exceedingly firm by the deposition of 

 chitin. Now, as the cells of the eye gradually increase in 

 length they will necessarily be subjected to pressure from 

 these membranes below, and from the cuticle, which they have 

 secreted, above. The cells of the eye, under these circum- 

 stances, will be able to retain their straightness for the greater 

 part of their length, which is absolutely essential for them, 

 only by growing in the direction of least pressure, i.e., towards 

 the middle of the disc ; hence whereas the cells in the middle 

 can remain vertical, those which are some distance from the 

 middle will have to take up a more oblique position, while 

 those at the circumference of the optic disc, where the cuticle 

 and somatopleure virtually adhere, will necessarily have to 

 take up a horizontal position if they are to develop at all; 

 in short, the degree to which the cells converge will depend 



(2) The nature of these membranes will be discussed later. 



