M. M. METCALF ON THE EYES AND SUBNEURAL GLAND OF SALPA. 319 



stolon. (Compare Fig. 7, Plate XLVII, and Figs. 1 to 10, Plate XLVIII.) 

 As in the solitary Cyclosalpa pinnata, the rod cells are the first to distin- 

 guish themselves, appearing at the same time in all regions of the eye. 

 Their cell boundaries become distinct. Soon they elongate, becoming 

 columnar (Plate XLVIII, Fig. 1). After a very short interval the deeper 

 staining of their inner ends shows the cell wall of this portion to be some- 

 what thickened (Fig. 5, Plate XLVIII, r). The adult histological con- 

 dition is reached by the greater elongation of the rod cells, an increase 

 in the size of their nuclei, a greater thickening of the cell walls of their 

 inner ends, and by a very dense deposit of pigment granules in the cells 

 of the pigment layer of the retina. This pigmentation does not show 

 in any of the young cyclosalpas still attached to the stolon up to the time 

 of the formation of the terminal wheel about to be set free. It must 

 then be deposited rapidly between the time when this wheel is formed 

 and the time when it is set free from the stolon of the solitary cyclosalpa. 

 During the change in the histological character and in the position 

 of the eye there is a concomitant change of form. The change from the 

 very early horseshoe-shaped ridge on the dorsal surface of the brain to 

 the thickened disk close pressed to the ectoderm and connected with the 

 brain only by nerve fibers, has already been noted. We have seen also 

 that the originally posterior edge of this disk curls over backwards to 

 form the rudiment of the second portion of the eye, seen in the anterior 

 end of the main body of the adult eye (e'" in Fig. 7, Plate XLVII, and 

 Figs. 1, 3, 5 and 6, Plate XLVIII). For a long time this rudiment is 

 connected to the main body of the eye by a considerable mass of cells 

 resembling the ordinary cells of the ganglion, i. e., by eye cells that still 

 retain their primitive character (Plate XLVIII, Fig. 9). When the wheel 

 of chain cyclosalpae is about to loose from the stolon this mass of cells 

 differentiates into three portions, one forming the pigment layer of the 

 anterior portion of the main body of the eye (Plate XLVIII, Fig. 8, p"), 

 another forming the pigment layer of the secondary part of the eye ; 

 the cells of the third portion becoming elongated to form the spindle 

 cells that bind the secondary part of the eye to the main body (Plate 

 XLIX, Fig. 1, q). While the differentiation of this mass of cells in the 

 three directions mentioned is taking place, the posterior end of the now 

 horizontal eye is dividing, longitudinally, in a vertical plane to form 

 the two posterior limbs seen in the adult eye. In Fig. 2, Plate XLVIII 

 (which is a horizontal section of that portion of the eye represented by 

 e' in Fig. 1, Plate XLVIII, at a corresponding stage of development to 



