371 



longitudinal fission (fig. 56). At times, though very rarely, 

 a third such cell can already be seen connected with the devel- 

 oping ommatidium. The most obvious feature of the develop- 

 ing compound eye at this stage, and for the whole of the 

 next day, is the closeness with which the cells are disposed, 

 making accurate observation of the development impossible 

 except in places where the cells have become artificially 

 loosened. 



This process of longitudinal division of the cells surround- 

 ing the ommatidia continues for a time after defaecation, till 

 four such cells are formed round each. In larvae eight hours 

 after they have defaecated this process is complete (fig. 57). 

 At this time there have also been formed, almost certainly 

 from these same outer-layer cells, a pair of rather small clear 

 cells, developed at the outer end of each ommatidium, and 

 generally very distinctly visible; they do not attach them- 

 selves as closely to the ommatidial cells as do the others. 

 The four long cells which embrace the ommatidia are the 

 developing pigment cells; during their formation from the 

 outer-layer cells their nuclei have taken up a more internal 

 position. The two small cells lying external to them will 

 become the lens cells. 



At this stage, then, the optic disc consists of a great 

 number of developing ommatidia, each consisting of a large 

 central basal cell, closely surrounded by seven sheath cells, 

 while at the distal end of each there are four vitreous cells, 

 and two lens cells outside these, adjacent ommatidia being 

 separated by the four elongated pigment cells which surround 

 these structures. 



The processes described so far have consisted almost 

 entirely in the cells adopting the position in which they 

 occur in the adult; visible differentiation has not proceeded- 

 beyond the rough assumption of size of the adult cells. The 

 remainder of the development consists of a change of the 

 general disposition of the optic disc as a whole (due chiefly 

 to an increase in the length of the cells), and of a partial 

 disappearance and gradual transformation of these almost 

 undifferentiated cells to the condition in which we find them 

 in the adult. 



The former process will be considered first. In the larva 

 in its first instar the imaginal disc of the compound eye is 

 very prominent, forming a definite thick area at each side of 

 the brain; as the larva gradually develops, the cells, as we 

 have seen, divide very extensively; hence the disc becomes 

 larger in area, but the cells, not having increased in size, are in 

 no way any more distinct; indeed, as the ectodermal cells 



