MUSEUM OF COMPARATIVE ZOOLOGY. 129 



it is possible to interpret the cells in an ommatidium as elements in a 

 thickened epithelium, all of which originally extended from one face of 

 the layer to the otlier, and the grouping of which is not even now in- 

 terfered with by any process of involution. But granting that the ret- 

 inal cells are thus arranged, it must be admitted that the surface on 

 which the rhabdomeres are produced corresponds to the sides of the cells 

 rather than to their distal ends. This interpretation of the position of 

 the rhabdome is not, so far as I am aware, contrary to any well estab- 

 lished facts, and indeed it is rather more in accordance with the condi- 

 tion seen in the eyes of some Arthropods than that implied in Watase's 

 theory. Thus, in the lateral eyes of scorpions the retinal cells are ar- 

 ranged as in an ordinary epithelium, and the lateral wall of each cell is 

 in part occupied by a rhabdomere. In this instance, then, it must be 

 admitted either that the rhabdomeres are produced on the sides of the 

 retinal cells, or that each cell has independently rotated upon itself, so 

 as to bring its morphologically distal end into a position corresponding 

 to the side of an ordinary epithelial cell. But there is neither direct 

 evidence to show that this rotation of single cells has occurred, nor, in 

 this case, can there be any motive assumed which might have induced 

 the rotation of single elements. I therefore believe that in the lateral 

 eyes of scorpions the rhabdomes are on the sides of the retinal cells in 

 the strictest morphological sense j and if they can occur in this position 

 in the eyes of scorpions, I can see no reason why they might not occur 

 in similar positions on the retinal cells of compound eyes. Hence it 

 seems to me as reasonable to interpret the retina in compound eyes as a 

 layer of modified epithelium unaffected by involutions, as it is to con- 

 sider it a layer in which each ommatidium represents an infolding. 



When, moreover, an attempt is made to show how a particular omma- 

 tidium has arisen by involution, some diflBculties are encountered. Thus 

 in Gammarus, in which the ommatidium is of a primitive type, each om- 

 matidial pocket would involve seven cells, two of which, the cone cells, 

 must be imagined as forming the neck of the involution, while the re- 

 maining five, the retinular cells, would constitute the deeper portion of 

 the pocket. The mechanical difficulty which would accompany the forma- 

 tion of an involution involving so small a number of cells must be obvi- 

 ous, and offers, I believe, an obstacle to the successful operation of the 

 process assumed in Watase's theory. 



The one instance in which Watase has described an actual involution 

 to form the eyes in Arthropods is the lateral eye of Limulus. These eyes 

 consist of a cluster of hypodermal pits, over each of which there is a cu- 



VOL. XXI — NO 2. 9 



