450 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



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The primitive arrangement of the ommatidia was probably in the form of simple tubes or 

 cylinders, with spaces between them and with rounded or indefinite facets. Mutual pressun- 

 among these tubular eyelets, arising from any cause, produces the hexagonal arrangement, the 

 most economical method so far as wall space is concerned. Interferences such as have been sug- 

 gested, as growth of individual ommatidia or increase in the number of ommatidia in the same 

 area, thus admitting a method of arrangement less economical of wall space, or the great increase 

 in length of theoinmati'liaand a relatively less increase iu width, attended by a progressive change 

 of the hexagons into squares (the apparent slipping of the rows of facets on one another), may 

 enter as factors into this change, but they do not suffice to explain all the conditions. It will be 

 understood, of course, that there are no changes in the individual facets, these remaining in the 

 same shape until they are cast off iu the moult. The changes which the individual ommatidia 

 undergo are very gradual, and since the number of cells for each omiuatidium is constant and deter- 

 mined at a very early period, excepting the accessory pigmeut cells, they must be attributed to 

 the change in the size and relative positions of the cells themselves rather than to intussusception. 

 It is possible, that the change from the hexagon to the square is not produced in the same way in 

 all cases and that the conditions of growth which bring about this result are far more compli- 

 cated than would appear from the suggestions which have been made. A careful study of the 

 arrangement of cells in the ommatidia of the eye of the young lobster during the period of transi- 

 tion would possibly throw some light upon this interesting subject. 



THE DEVKI.OPMKNT OF THE COMPOUND KYK. 



Five years ago (20) 1 stated my conviction that the compound eye of Alpheus, and probably 

 also of Palaemontes and of the large Isopod, Ligea oceanica, originated I rom a thickening of the 

 superficial ecloblast. The development of the eye iu Alpheus was more fully described in a pre- 

 liminary notice (131). 1 will now recapitulate the main results, at the same time correcting such 

 errors as I have detected. 



In studying the development of the eye the following are some of the subjects which present 

 themselves for investigation: The origin and structure of the optic disks; the separation of the 

 optic disks into a ganglionic and retinal portion by an intercepting basement membrane; the dif- 

 ferentiation of the retina into ommatidia or eyelets; the differentiation of the optic ganglion and 

 the development of the optic nerve, by means of which the sensory end organs of the retina come 

 in direct relation with the ganglion. 



(1) Origin of the Optic Disk. The optic disks (Fig. 58, PI. xxxn) consist of large ectodermic 

 areas or patches on either side of the middle line. They are centers of rapid cell division, united 

 by means of the lateral cords, which are bauds of proliferating cells, with the thoracic-abdominal 

 plate. 



So far as 1 am aware we have no account of the origin of the optic disk in any Decapod except- 

 ing Alpheus, Astacus (54), Crangou (30), and Homarus (47\. Parker, in his careful studies on the 

 eye of the lobster, was unable to obtain the earliest traces of the developing optic disk, and the 

 accounts of Reichenbach and Kingsley differ very materially. I will therefore describe somewhat 

 in detail the process by which the optic disk is produced in Alpheus. 



The optic disks at the time when they consist of a single stratum of cells are shown in Figs. 

 58, 08, and 69. A series of four transverse sections through the central portion of the left optic 

 disk is represented iu -Figs. 64-67. The posterior face of each section is presented, the series pass- 

 ing from the front backward. 



Since it is from the optic disk that the eye and its ganglion are developed, the important inquiry 

 which arises at this stage is, how is the change effected by which the disk passes from this single- 

 layered to a many-layered condition like that seen iu the egg nauplius (Figs. 107, 114)? If the 

 eye represents a series of hypodermal pits, it would be reasonable to look for some trace of these 

 infoldings in the embryo. If, on the other hand, the compound eye of the higher Crustacea repre- 

 sents a closed vesicle produced by a single iuvagiuatiou of the hypodermis, of the type seen in the 

 pi -ototracheate Peripatus, we should expect to find some trace of an involution at this stage. So the 

 answer to this question may have an important bearing upon the phylogeny of the compound eye. 



Cell boundaries are easily discernible at the surface, but it is evident that the nuclei do not 



