313 S. WATASE. 



depigmented, witlioxit whicli tlie process of tlio formation of tlie 

 omnmtidiii and tlie increase of the ocular area is perfectly unin- 

 tellii;it)le. Those cells in the figures which have no {granular 

 structure are, in the state of nature, charged with a larir(^ quan- 

 tity of pigment granules, and those large cells forming the walls 

 of the lateral folds are granular and free from pigment granules. 



Transverse sections are not fitted for the study of the forma- 

 tion of the nerve fibres. In this stage, as in the preceding ones, 

 each ommatidium sends out nerve fibres from its basal end 

 towards the interior of the body. Besides the cells in the bottom 

 of each iimmatidial pit, each gigantic cell of the outer walls of 

 the dorsal and ventral folds sends out a nerve process from its 

 basal end, which runs inward and forward, joins the nerve fibres 

 from the bottom of each ommatidium, forming a large bundle, 

 and goes to the brain. 



The invagination of the lateral and posterior margin of the 

 ocular area is usually not permanent. With a growth of the 

 animal and an increase of the surface of the ocular area, the 

 invaginated folds, both on the sides as well as behind the ocular 

 area, become stretched out in the adult animals and leave no 

 trace of their existence. Occasionally, however, one meets in the 

 adult with a remnant of the involuted tube in the posterior part 

 of the eye, which exists as a ball of pigment cells buried in the 

 midst of the mesodermic tissue. 



Tlie history of the ommatidial cells in the compound eye of 

 Limulus may then be stated in the following way : 



(1). A stage of undifferentiated ectodermal cells. 



(2). Thickening of these undifterentiated cells, accompanied 

 with the invagination beneath the surface. 



(3). Extreme enlargement of the invaginated ectoderm, fol- 

 lowed by the acquisition of pigment granules and by division. 



(4). Pushing out of the resulting small pigmented cells towards 

 the surface where they differentiated into two fundamental 

 groups: (a) Ordinary epithelial cells, which secrete the chitinous 

 cuticle over the ocular area, including the lens, or pack the inter- 

 spaces of the neuro-epitheliuTu ; {h) neuro-epithelinm, consisting 

 of the rod-bearing retinulae and the central ganglion cells, the 

 two forming the sensory portion of the ommatidium. 



