208 ANDREWS. [Vol. VII. 



rapidly as the vesicle, but lies near the cornea, so that a space 

 is left between it and the retina. This space is lined next the 

 retina by a layer of retinal rods, which appear with the pigment, 

 and perforate it. At first, there is a mass composed of a few 

 rods, to judge from the figures, which then becomes split up 

 into as many rods as there are cells ; meanwhile, the pigment 

 spreads from one point as a thin layer where the rods and retinal 

 cells are united. There still remains a large space between 

 the rods and the lens, represented in the adult Asterope by the 

 coagulated material in Fig. 28, 9-5. 



The formation of the vitreous body filling this space is the 

 most unexpected part of the whole process. It is secreted by 

 a single large cell. This is found in the brain before the retina 

 has begun to form at the surface, but later it introduces itself 

 into one side of the retinal vesicle, and subsequently breaks 

 through its walls, so as to pour a secretion into the central 

 cavity. This secretion passes out from near the enormous 

 reticulated nucleus through the cell to the above space, and 

 forms all the vitreous body between the lens proper and retinal 

 rods. 



Such unicellular " glaskorperdrusen " are said to occur in 

 other annelids with highly developed eyes. 



That these bodies have a remarkable appearance is indicated 

 by the fact that Graff (1) found and figured them as auditory 

 sacs, one near each eye in the adult, connected by a nerve with 

 the circumoesophageal commissures, and containing an otolith. 

 Yet he recognized that each has at first sight somewhat the 

 appearance of a cell. 



Since the Alciopidae are at once among the few parasitic 

 annelids, and possess the largest and most complex eyes, we 

 may assume that there is considerable chance of secondary 

 changes having taken place, so that their present ontogeny 

 does not repeat the primitive method. Thus, the formation of 

 the lens inside a closed sac, and the secretion of a large part 

 of the refracting material by the action of a single cell having 

 no direct connection with the retina, may well be recent inno- 

 vations, and offer no objection to the acceptance of other 

 methods as the more primitive and general. 



