318 JOHNS HOPKINS UNIVEKSITY MORPHOLOGICAL MONOGRAPHS. 



sented in Figs. 1 to 3, Plate XLVII, and have always found the same 

 appearance of a definite, though slightly developed, horseshoe-shaped 

 ridge. This appearance is so constant and uniform that we can safely 

 say the eye of the chain cyclosalpa passes through a stage when it cor- 

 responds in shape to the eye of both the adult and embryonic solitary 

 Cyclosalpa pinnata. The central cells of the ganglion and the cells of 

 the core of the ridge degenerate at the same time, as is also the case in 

 the solitary cyclosalpa. 



Very soon the horseshoe-shaped arrangement of the eye cells is lost. 

 They are, from the first, close pressed to the ectoderm. Soon after the 

 appearance of the optic ridge the ectoderm arches up over the ganglion, 

 carrying with it the cells of the ridge. The commencement of this pro- 

 cess is seen in Fig. 5, Plate XLVII. The eye cells lose their connection 

 with the ganglion, except that the non-cellular core of the ridge is pulled 

 out into long fibers that bind the eye cells to the ganglion. The rudi- 

 ment of the eye is now a thickened disk of cells, close pressed to the 

 ectoderm, with fibers connecting the center of its ventral face with the 

 non-cellular core of the ganglion. As development proceeds the anterior 

 edge of the disk approaches the brain till it comes in contact with it 

 (Plate XLVII, Fig. 7). While the anterior edge shifts its position, the 

 posterior edge retains its former place, the disk becoming in this way 

 nearly perpendicular to the dorsal face of the ganglion (Plate XLVII, 

 Fig. 7, and Plate XLVIII, Fig. I). The nerve fibers which, from the 

 first, connected the center of the disk with the non-cellular core of the 

 brain, now, of course, lie along the posterior face of the perpendicular 

 disk (Plate XLVII, Fig. 7, and Plate XLVIII, Fig. 1, on}. During its 

 change of position the disk remains in connection with the ectoderm, 

 causing the latter to approach anteriorly almost to the surface of the 

 ganglion. The posterior edge of the disk, during this shifting of position, 

 curls over backwards, forming the first rudiment of the second mass of 

 retinal tissue which lies in the anterior curve of the main body of the 

 adult eye (e'" in Fig. 7, Plate XLVII, and Fig. 1, Plate XLVIII). The 

 whole eye continues to bend forward till it comes to lie horizontal with 

 the originally anterior edge of the disk posterior and the originally 

 posterior edge anterior. (Compare Figs. 1, 5 and 7, Plate XLVIII.) 



The histological differentiation of the retinal elements, the change of 

 form of the eye and its shifting from a perpendicular to a horizontal 

 position, proceed simultaneously, all three processes becoming complete at 

 the time when the circle of chain cyclosalpas is set free from the mother- 



