MUSEUM OF COMPARATIVE ZOOLOGY. 199 



thicker around the whole circumference of the cell, and take on a lus- 

 trous appearance. With Kleinenberg's hsematoxylin the substance of 

 the rhabdomeres can be colored, and the line of demarcation between 

 products of the separate cells can be distinguished. This structural con- 

 dition can be traced to the deeper part of the retina, where the cell 

 outlines become indistinct, the rhabdomeres incasing each retinal cell 

 for a half or two thirds of its length. 



Pigment (PI. III. fig. 19, gra. pig.) is uniformly distributed through 

 the retinal cells, as well as the perineural cells to be described later. This 

 is best seen in sections perpendicular to the axis of the eye. Phao- 

 spheres, although present in the median eyes, do not occur in the lateral 

 eyes. The optic nerve (PL III. fig. 18, n. opt.) emerges from the deep 

 end of the retina, and its course is so oblique to the axis of the eye that 

 a section which shows the retina well seldom shows much of the optic 

 nerve. 



The perineural cells surround the depressed retinal area, and their 

 attenuated ends, especially on the ventral side of the eye, often reach 

 out, even in the adult condition, in front of the retinal cells themselves 

 (PI. III. fig. 18). The positions that the nuclei occupy in the ventral 

 portion of the perineural ring suggest that these cells may at one time 

 have extended far enough to have completely covered the retina, and the 

 fact that in young individuals (PI. III. fig. 21) the retina is largely cov- 

 ered by the perineural cells indicates that in all probability the lens is the 

 product of these cells. In that event the perineural cells are the physio- 

 logical equivalent of the lentigen. The peripheral margin of this lentige- 

 nous ring passes by insensible gradations into the surrounding hypodermis. 



The development of the lateral eyes is referred to by Kowalevsky and 

 Schulgin ('86, p. 531) as follows: "Die Seitenaugen entwickeln sich 

 unabhangig von den Mittelaugen, und bei ihrer Ausbildung nimmt die 

 Vertiefung der obern Schicht der Kopfplatte Antheil. Die Einzelheiten 

 dieses Vorganges sind von uns noch nicht bearbeitet." This is the only 

 reference which they or other students have made to the development of 

 the lateral eyes. 



The " ocular areas," as Lankester and Bourne designate the regions 

 occupied by the lateral eyes, appear in Centrums as pigmented tracts of 

 hypodermis on either side of the head and a little below and behind the 

 median optic sacs. Horizontal sections of the embryo cut these areas in 

 tlie most advantageous way for a general study ; they show that the 

 whole ocular area is produced by a thickening of tiie hypodermis. 



