312 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



cyclosalpa eye as double ; the two halves, right and left, being connected 

 by a posterior transverse band. This description is a little misleading, 

 for the size of the posterior curved part of the retina and optic ridge is 

 fully as great as, and in most species a little greater than that of either 

 of the two anterior limbs, and it is an equally important part of the eye. 

 The eye forms a continuous curve a little greater than a semicircle, and 

 is a single structure in its adult form and in its origin. In the chain 

 form of different species the type is departed from and the eye is in 

 two cases strongly bilobed, as will be shown further on. 



The ridge increases in size as the ganglion grows. As the central 

 cells of the ganglion degenerate, the cells of the core of the ridge also 

 degenerate ; the two areas not being separate, but being, from the first, 

 continuous with each other. Fig. 5, Plate LI, represents a section of the 

 ganglion and eye at a time when the degeneration described has made 

 some progress. It is a vertical transverse section through the central 

 part of the ganglion. On the dorsal surface are seen sections of the two 

 anterior limbs of the developing optic ridge. Fig. 6 shows a more 

 posterior section of the same series, cutting the posterior curved portion 

 of the horseshoe-shaped ridge. 



At a considerably later period the retina cells begin to assume their 

 characteristic appearance. The first noticeable change is the enlarge- 

 ment of the most peripheral layer of cells over the dorsal portion of the 

 ridge. They elongate and become columnar, with their long axes dorso- 

 ventral (Plate LI, Fig. 7). Ventral to the rod cells there are about three 

 times as many cells having the character of the ordinary ganglion cells. 

 About one-third of these will remain unmodified to form the inter- 

 mediate cells of the retina. At about this time the other two-thirds 

 begin to become pigmented, and, a little later, the walls of the inner 

 third of each rod cell become thickened and so stain more deeply. The 

 adult condition is reached by an increase in the size of the rod cells, by a 

 greater thickening of the walls of their inner ends, by a greater deposit 

 of pigment in the inner cells of the retina, and by a shifting of the retinal 

 area from the dorsal surface to the dorsal part of the inner surface of 

 the ridge. (Compare Fig. 7 with Fig. 8, Plate LI.) This change of 

 position seems to be caused by the greater growth 'of the outer face 

 of the optic ridge, pushing dorsalward that edge of the retina which 

 was most distant from the mid-dorsal point of the brain (p in Fig. 7, 

 Plate LI). Of course, by this shifting the long axes of the rod cells, 

 which were originally vertical (Fig. 7), become horizontal (Fig. 8). 



