452 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Wauderiug cells are seen in contact with tbis inetnbraue (Fig. 13(5), Vmt they prohahly do not 

 share in its secretion, although they occur in the closest relations with it. 



At a little later period (Fig. 138) the retinal portion is several cells thick on the outer edges 

 of the lobe, while it is a single stratum iu the sagittal section, shown in Fig. 138. The plane of 

 section is near the center of the lobe. The deeper nuclei of the ganglion are large and clear, the~ 

 outer are smaller and stain more intensely. This section can be clearly understood if compared 

 with the transverse section, Fig. 146. We see that the optic ganglion is here divided into an 

 external or distal part and an internal or proximal portion liy a thin sheet of very large and clear 

 ganglion cells. Parker (47) describes and figures an exactly similar structure iu the lobster, and 

 1 fully agree with him in regarding this band of nuclei as representing similar bauds, which 

 Keichenbach (Taf xii. Figs. 173, 174,^4. \V. I. ]V.) describes in the crayfish. In Keichenbach's 

 plates these nuclei apjiear as a nariow fold, forming the lining of what is describetl as a secondary 

 optic invagination. 



Three iiuiict-substanz mas-ses have already appeared in the inner iialfof the optic ganglion 

 the external middle segment, which lies next to the band of large nuclei, and the internal middle 

 and proximal segments. The proximal medullary mass is much the largest and is the first to be 

 differentiated, although the others follow close u])on it. The dividing nuclear band lasts but a 

 short time, and in Stage 10 (Fig. 107) has disappeared. In front of it is developed the lame gang- 

 lionaire or distal segment of the optic ganglion. This has an outer convex surface which is con- 

 centric with the basal membrane and with the outer surface of the retina, and it is carpeted by a 

 special layer of ectoderm cells. These apiiear in section as a single row of elongated nuclei. 



In an early communication (20) I stated my belief that the puuct-substanz arose from a 

 metamorphosis of ganglion cells. This view was suggested by certain appearances presented 

 by the large clear nuclei, more particularly by those of the dividing band in the optic glangliou 

 (see Fig. ISO). Kingsley (3l'),came to the same conclusion in regard to certain large clear nuclei 

 iu or near the distal segment of the optic ganglion of Crangon. In reviewing this subject more 

 carefully I am convinced that this interpretation is erroneous. These large clear cells are iu 

 reality undei'going indirect cell division, as proved by the karyokinetic figures which are occa- 

 sionally seen. Both tbe chromatin network and the chromosomes are exceedingly delicate, and 

 when the section is in the plane of the equatorial plate an appearance is presented which under 

 certain conditions of staining and preparation might easily be interpreted in favor of retrogressive 

 metamorphosis. I conclude that the punct-substauz of the nervous centers is in all cases derived 

 from the protoplasm of cells, not from cell nuclei. 



In Stage ix when eye j)igment first appears, the structure of the retina is very simple. By 

 the transverse section (Fig. 146), we see that the retina consists of a thickened ectoderm plate, 

 thickest in its deeper portions; thinning out toward the middle line at the surface. It has the 

 shape of the half section of a concavo convex lens. 



In Pahemonetes the structure of the eye is i)recisely similar at this stage. Fig. 189 gives a 

 section of the eye of this prawn, on a line with the oesophagus behind the optic ganglion, where 

 it is seen to rest against the yolk. The black pigment, though appearing to arise in connection 

 with certain inesoderinic cells (wandering cells from the yolk), it in reality belongs to deep ectoderm, 

 and marks the retinular cells. The cell protoplasm bearing the pigment bodies grows outward 

 (Fig. 146), and also pierces and extends some distance below the basement membrane (Figs. 191, 

 ]92). The latter is a delicate cuticular structure secreted by the ectoderm cells which lie along 

 the line of division of retina and ganglion, and continuous with the baseu)ent membrane of the 

 hypodermis. In some sections it appears to be duplex, a condition described for the eye of 

 the lobster by Parker (47), iu which the inner layer enfolds the optic ganglion. The wide ojien 

 fissure which now exists l)etweeii retina and ganglion (seen in transverse .section at au, Fig. 136) 

 is partially tilled with yolk. There is not the slightest doubt that cells enter this fissure from 

 the yolk (Mes. Fig.s. 146, 167, 189, 194, etc.), but what the fate of these wandering bodies i.s, I find 

 it very difficult to decide. I think, however, that it can be stated definitely that none of the.se 

 cells enter the retinogen. They mast supply .some of the pigment found in this region, or they 

 may become converted into connective tissue. In Stage x (Fig. 167) the yellow accessory pig- 

 ment cells are clearly diflerentiated, or at least the pigmeut which these bodies give rise to, is seen 



