VISUAL TISSUES 249 



nerve branch, and that these or others wind in a spiral through the cyto- 

 plasm around the nucleus of these nerve cells, and then pass out inwardly 

 to send branching ends to the visual cells (Fig. 220). Miss Hyde does 

 not show an outer (one cannot say as yet "proximal" or "distal") area 

 of cytoplasm on these external ganglion cells, as Hesse and others have 

 pictured, in an analogous position, in so many other pecten eyes and as 

 the writers saw in Pecten tenuicostus. Some faint indication of it is 

 shown in one of her figures. 



This edge shows that the cells of this layer have some very peculiar 

 nervous function besides their connection with the visual cells as de- 

 scribed by Miss Hyde and as weakly shown by certain fibrils in the figures 

 of Patten and Hesse. The edge is drawn out into thick groups of rod- 

 like processes, which converge toward an outside central point and join 

 the upper branches of the optic nerve. In doing this, it can be seen, 

 they come to act as intermediate cells between the visual cells and the 

 central ganglion. These visual cells also have a direct connection by 

 means of the lower or lateral branches of this nerve, so that there are two 

 different pathways for the impulse, and we have a case where communi- 

 catory nerve cells have entered the retina as they had begun in the worm, 

 Vanadis, where they formed a ganglion below the retina and outside of the 

 eye. This layer of cells probably has some function to perform which is 

 analogous to that of the layers of ganglion cells in the eye-stalk of the 

 Arthropoda or the ganglion layers of the vertebrate retina. 



Beneath the retina, and separated from it by the wide space filled 

 with the vitreous fluid, is the tapetum, a layer filled with refracting 

 granules and used apparently to reflect the light. This layer is formed 

 by a single, wide, thin cell in other pectens, and the same will probably 

 be found true in the form we are studying. Underneath the tapetum is 

 the pigment layer, a simple epithelial covering whose thick cubical cells 

 are filled with red pigment granules. 



The dibranchiate cephalopod eye is probably one of the most complex 

 in existence, although this complexity is more easily understood and more 

 superficial than in the vertebrate eye. The eyes of both of these groups 

 should be studied from the developmental point of view to properly 

 understand their histology. 



The eye of the squid, Loligo Pealii, begins as an invagination which, 

 as in so many other mollusks, becomes the visual sac (Fig. 221). At 

 an early period this sac becomes cut off from the exterior, and develops 

 the posterior part of its lining epithelium into the visual epithelium. 

 The anterior region develops on its inner surface a rather larger part than 

 one half of the lens. This lens is a homogeneous cell-product which is 

 formed as a series of very perfect lamellae lying parallel to each other. 

 The ectodermal surface, which faces outward from this inner lens epi- 



