MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 451 



all lie at the same Ii-vel. Passing now to Stage IV (Fig. 72), we notice several important changes 

 iu the external api)earauce of the optic disks. They have approached nearer the iniildle line; 

 they have increased in area; their nuclei are more crowded ; near the center of the disk and on 

 the side of it toward the middle line (Fig. 7L', C. M.) the nuclei are distinctly larger. In inter- 

 preting these changes we must resort to sections and a careful study of dividing nuclei. A com- 

 plete series of consecutive transverse sections through the- left optic disk at this critical stage is 

 given iu PI. xxxv. The last of the series cuts the rudiment of the first antenna. Karyokinetic 

 figures come to our aid in pointing out the way in which the growtii of the disk is eflectcd. Tiie 

 nuclei divide either radially, the plane of division, clearly marked by the equatorial plate, being 

 perpendicular to the surface, or horizontally, the plate being in this case parallel with the 

 surface. In a single I'isk at this stage there were eight cells undergoing radial division. Of 

 these, six were near the periphery, where the cells formed a single stratum, while two were near 

 the center, where the disk was slightly thickened (Fig. 8(t, G. M.). It is evident from this and 

 similar cases that the increase iu area of the disk is accomplished by radial cell division. The 

 same is true of all jjroliferating areas in the lateral cords, where the appendages are soon budded. 



In the area marked ('. M. (Fig. 80) the optic disk is no longer a single layer. This thickening 

 is due either to horizontal cell division, that is, delaniination, or to emigration. The appearances 

 of emigration are often very deceitful, bu t I think we may safely conclude that the initial thick- 

 ening of the optic disk in the proliferating area, marked C. ^I. (Figs. 80, 90), is due to emigration, 

 that a solid ingrowth akin to invagination takes place at this point. Thus the cell marked ec in 

 Fig. 80 is distinctly below the surlVice. The boundaries of the cell can be clearly seen. The cell 

 ec in Fig. 90 (dotted line should be extended), on the other hand, is clearly iu contact with the 

 surface by a slender protoplasmic jjrocess, while the nucleus lies at a much lower level. I inter- 

 pret the latter as a cell at the point of breaking all connection with the surface and migrating to a 

 lower position. In the lirst instance this has already been accorapWshed. 



Iu the stages under discussion there are one or two cases out of a large number of sections 

 involving the optic disks of several individuals, which probably indicate delaniination in the 

 peripheral parts. At a later jieriod (Figs. 102-107) the nuclear figures are conclusive. In one 

 instance (Fig. 102, ec.) two cells are seen delaminating side by side. The thickening of the optic 

 disk is thus due in part to delaniination, apd this process is i>robably supplemented by emigration, 

 at least in the central area. The central area represents in all probability the " optic invagination" 

 of the crayfish, and is concerned solely with the production of the optic ganglion. In Aliiheus 

 there is a proliferating area simply, but no superficial depression or invagination in the strict 

 sense. 



It is noticeable that in Stage iii (PI. xxxiii) wandering cells, or cells which travel through the 

 yolk, have not appeared in the neighborhood of the optic disks. In Stage TV (Fig. 70, Y. C.) they are 

 ' not far away, and in later stages (Fig. 91) these cells are close upon the disks. Some of them which 

 enter this region, coming near to or uniting with the disks, undoubtedly degenerate (coniiiare 1'. C, 

 Fig. 94, .s-.s'. Figs. 95, 90, 99, 100). Some of the cells of the disks next the yolk elongate, and appear 

 to form a somewhat transitory covering. As already suggested, these bodies are probaVily derived 

 from the wandering cells. 



The various stages by which the disk, already described, is converted into a conspicuous, 

 lobular mass of cells in closest relation with the antennular ganglion such as we have in the egg- 

 nauplius (Fig. Ill, 0. L), can be seen by reference to the plates (Pis. xxxvi-XL). 



(2) The Development of the Retina and Optic Ganglion. — The next event of importance is the 

 differentiation of the optic disk into ganglionic and retinal portions. This is already begun iu 

 Stage VII. A deeper layer from which the ganglion is developed ( Figs. 129, 132, G. L.) is gradually 

 separated from a sniierticial tier of cells with very large nuclei {OE). This layer is the retinogen. 

 The differentiation begins in the lateral lower halves of the optic disks and extends upwards and 

 toward the middle line. As the <lisk s[irca(ls outwards, and at the same time increa.ses iu thick- 

 ness, it tends to overgrow the hyi)oilermis and becomes raised into a lobe or fold. The optic lobe 

 (Figs. 136, 138) thus repre.sents a thickening of the hypodermis. It is covered next the yolk by a 

 delicate basement membrane (-Bm.), which is continuous with that of the surrounding hypodermis. 



