528 THE SENSES AND SENSORY ORGANS. 



\\Vismann [2], whilst he admits that four nuclei appear in 

 the cell from which the corneal lens is developed, says there is 

 only one cell, and not four one cell with four nuclei. Hickson 

 says that ' Semper's nuclei' have nothing whatever to do 

 with the formation of the corneal facets ; but these ' are 

 formed from a continuous layer of protoplasmic substance 

 which underlies the whole surface of the cornea, and remains 

 as a living protoplasmic lamina until the eye is fully developed, 

 and then shrivels up ' [237, p. 23]. Such a statement is 

 surely an anachronism in a paper published in 1885, it would 

 have looked better in one written in 1850. In every case 

 when such continuous nucleated layers have been described, 

 subsequent observations have dispelled the illusion. 



Grenacher describes the cornea as nearly flat on its inner 

 surface. I have frequently seen sections in which this is the 

 case, but they are ill preserved. Specimens which have been 

 kept for some time in spirit are usually in this condition. 

 The convexity of the inner surface of the cornea is only 

 preserved in specimens fixed in osmium peroxide. He also 

 represents the remains of the membrana propria of the cone as 

 the pseudo-cone [222, Fig. 63]. In other respects our de- 

 scriptions and figures are very similar, indeed, almost identical. 

 Grenacher's specimens, he says, had been preserved in alcohol ; 

 evidently without any special precautions, as he observes: ' I 

 came across specimens of M. vomitoria, preserved amongst 

 numerous others in alcohol ( ]}'cini^cis(), which had lost their 

 pigment and had become white ' [222, p. 90]. It was from 

 these he drew his figures and description. 



The Cone. The cone of the ommateum in the Blow-fly is 

 pseudo-conic. On the seventh or eighth day of the pupa 

 state, however, it consists of four transparent cells, Grenacher's 

 aconic stage. 



The pseudo-cone in the imago (PI. XXXVI., Fig. I, c, c) 

 exhibits no trace of structure ; it is surrounded by a thin mem- 

 brana propria, and by several large, flat epithelial cells, loaded 

 with bright crimson granular pigment. This pigment assumes 

 a brown colour in chromic acid and alcohol. 



