SENSORY ORGANS. 171 



the crystalline cone-cells into groups of four in the subjacent layer, while, in the 

 lowermost layer, the cells of the retinulae form bundles of seven each, these 

 bundles reaching and enclosing the lowermost, pointed ends of the cells of the 

 crystalline cone. The separate rudiments of the ommatidia are divided by 

 numerous undifferentiated ectoderm -cells. 



In this last point the accounts of Paekeu and Hereick differ. In Hornarv.s 

 it is said (Parkek) that the separate rudiments of the ommatidia lie close to 

 one another, and are not separated by any kind of intermediate pillars. Three 

 layers can be distinguished ; from the outermost arise the corneal hypodermis- 

 cells and the anterior pigment-cells (distal retinulae), from the median layer 

 the cells of the crystalline cone, and from the lowest the actual retinulae.* 



The paired lateral ej^e of the Isopoda also develops in a similar way. In 

 Bullar's account of Cymothoa (No. 81) the rudiment of the eye stands in close 

 connection with that of the optic ganglion. The two proceed from one and the 

 same ectodermal thickening. While the inner layers of this thickening become 

 detached for the formation of the optic ganglion which is connected with the 

 brain, a suiaerficial hj'podermal thickening becomes marked off by a pigmented 

 basal membrane. This hj'podermal thickening represents the rudiment of the 

 eye, in which the separate ommatidia seemed to be marked off from one another 

 by a strongly pigmented mantle of cells. The details of the development of 

 the ommatidia were not followed in this case. 



In the cases we have hitherto described, the development of the 

 eye is comparatively simple, but in Astacus (Eeichenbach, ^o. C5) 

 and Crangon (Kingsley, Ko. 52) complication occurs through an 

 invagination Avhich forms on the boundary between the rudiment 

 of the eye and that of the gangHon. This invagination, which, 

 according to Eeichenbach, is replaced at a certain stage of develop- 

 ment by a more solid ingrowth, gives rise to the opiic fold between 

 the rudiment of the eye and the gangliogen ; in this fold an inner 

 and an outer layer are recognisable. Although Eeichenbach did 

 not closely follow the future fate of the optic fold, it appeared 

 probable to him that the outer layer enters into the formation of 

 the eye and yields the retinular layer, while the inner fold enters 

 into tlie formation of the optic ganglion. It was pointed out, 

 especially by Carriere (iSTo. 44), that in such a method of develop- 

 ment of the retinular laj^er the position of its cells is reversed, their 

 basal ends being directed towards the cells of the crystalline cone, 

 and their upper ends towards the rudiments of the ganglion, and 

 that we must thus assume a later re-arrangement in the retinular 

 area, such as takes place among the Araneae, but has not yet 

 been observed among the Crustacea. The suggestion made by 

 Patten appears to us probable, viz. — that the optic fold has nothing 

 to do with the formation of the eye, but merely yields the material 



*' [Cy. Parker, G. H., Retina and Optic Ganglia in Decapoda, Mitthdl. Stat. 

 Zool. Neapel., xii., 1895.] 



