ON THE DEVELOPMENT OF LIMULUS LONGISPIXA. 77 



It i.s interesting that the lateral compound eye of Liniuliis is at 

 first an epiblastic thickening with an invagination like a simple eye, 

 and not a group of thickenings or invaginations, and tliat it ap]iear.s 

 at the outer margin of the Lrain, as in the cnse of otlier Arthropods. 

 The lateral eyes of Lininlus are not thoracic eyes, hut cephalic eyes. 

 After their ibrmation they shift their position gradually dorsally and 

 posteriorly and at last they are behind the lateral hump, at tlie level of 

 the fourth a[)])endnge (fig. 17). 



Cells forming the bottom of the invagination of the lateral eye 

 become large and are faintly stained ])y a colouring solution (fig. 62). 

 The invno-ination is faintly divided towards the posterior end into two 

 brariches or depressions, ventral and dorsal. The large faintly stained 

 cells are found in the ventral branch only. The dorsal branch is very 

 short and is soon aborted. The invagination of the lateral eye is not 

 deep, and its bottom prolonged anteriorly as a short solid tube 

 (tig. 63). 



After the disappearance of the dorsal branch of the invagination, 

 cells forming the thickening, dorsal to the invagination, are pigmented 

 îind grouped into rudimentary ommatidia (figs. 64-66), smaller and 

 more pigmented cells surrounding larger and less pigmented cells. 

 Each of these groups of larger cells is spindle-shaped. Thus at this 

 time there is not to be found a separate invagination for each ommati- 

 dium. The first differentiation in the lateral eyes is the grouping of their 

 constituent cells into rudimentai-y ommatidia. I do not consider the 

 invao-ination for an ommatidium as of much value, as it seems to me 

 to be produced sec<3ndarily by the thickening of the cuticula into the 

 lens. The changes which take place in the lateral eyes after the for- 

 mation of the rudimentary ommatidia are not minutely known to me. 

 The optic nerve of the lateral eyes is formed from the lengthening 

 of cells forming the solid proLjngation of the invagination (fig. 6(!), 



