THE DEVELOPMENT OF THE COMPOUND EYE. 551 



therefore, if I am right, corresponds with the vitreous in the 

 Vertebratse. 



Multi-nucleate Rhabdomes. Although the rhabdomes of the 

 Blow-fly only exhibit one nucleus in the shaft (PL XXXVIII. , 

 Fig. i) there is no doubt whatever that in many Arthropods the 

 rhabdomes are multi- nucleate cells. Grenacher's figures are 

 sufficient evidence of this, and I would draw attention to the 

 strong tendency of the parablastic elements of the mesoblast 

 to become multi-nucleate. Although I have not been able to 

 satisfy myself that this condition is characteristic of any stage 

 of development in the Blow-fly, it is very possible that it is. 

 Amongst the crowded nuclei at the outer end of each rhabdome 

 indicating the pigment cells of the irides and the chamber, it 

 is impossible to say that some of these do not lie within the 

 great rod; indeed, some appear as if they do. When several 

 nuclei exist in one great rod, portions of the original cell may 

 become independent cells, as they certainly do in Tipula. 



The Origin of the Pseudo-cone. The pseudo-cone is formed 

 from four cells which appear beneath the cornea. Some 

 observers believe these cells are the four sub-corneal cells ; this 

 is the opinion of Grenacher, but Patten strongly advocates 

 the view that they form a second layer of cells beneath the 

 sub-corneal cells. I am very much inclined to think they do. 

 It appears to me that the sub-corneal cells are converted into 

 the corneal lens, and that their shrivelled nuclei persist as four 

 minute nuclei, which are always present at the edges of the 

 corneal lenses ; I am not, however, in a position to state this 

 with certainty. 



The pseudo-cone is undoubtedly formed by the vacuolation 

 of four cells, which are not apparent until a late stage of de- 

 velopment, and it appears to me possible that these four cells 

 are developed from the anterior end of the great rod by the 

 separation from it of four nucleated portions of protoplasm. 

 If so, then the pseudo-cone, which encloses the anterior end 

 of the rhabdome, is also mesoblastic in origin. Certainly the 

 cells of the pseudo-cone have nuclei, which are very unlike the 

 four sub-corneal nuclei of an earlier stage. 



