298 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



origin of the unpaired median-eye nerve, and nearly opposite the position of the median-eye lobe. 

 The anterior division of the lobe* is rounded, somewhat pear-shaped, and more than twice as thick 

 as the base of the nerves to which it gives rise (PI. I, Fig. 2). Each lobe is mostly made uj) of a 

 tangled mass of nerve fibers ijroceeding from small ganglionic cells (chromatic cells) buried in 

 the mass of the lobe (PL I, Figs, 1, 2, PI. II, Figs, la, th; and young, VI. Fig. 1, PI. VII, Fig. 2, 

 which represents the entire lobe, and forming an irregular cylindrical sheet, or plate, passing 

 through the lobe.t This layer is not, then, wholly a cortical one, but is in part internal, and may 

 be called the chromatic-cellular layer of the lobe, and penetrates the lobes from beneath, being an 

 offshoot from the mass of chromatic-cell layers impinging upon or partly enveloping the lower side 

 of the lobes, as seen in PI. VI, Fig. 1, cli. c. layer. In the young Limulus this layer is more external 

 and cortical. These cells differ from the ordinary large ganglionic cells in having very little 

 protoplasm enveloiiing the nucleus, the latter taking a deep stain, and thus being very distinct, 

 while the nuclei take the carmine stain well. They might be called aprotoplasmic ganglion-cells. 

 These cells appear to be identical with Saint Eemy's chromatic cells. Many of these cells are 

 scattered through the center of the lobe, especially in the lower portion. We shall refer to them 

 as chromatic cells, or aprotoplasmic or small ganglion cells. We are not entirely satisfied with 

 the term chromatic cells, as this does not describe their more fundamental nature or their small 

 size. Similar chromatic cells are scattered throughout the inner aspect of the lobe, between the 

 periphery and the chromatic-cell layer within. 



These chromatic cells are ])lainly seen in the section represented by PI. II, Figs. 7, 7«, 

 to be unipolar and to give off the flue fibers which form the tangled fibrillar mass constituting 

 the greater part of the lobe. But besides these fibers throughout the tangled mass are to be seen 

 irregular masses of the white myeloid or punctured substance {punMsubstanz) which appears to 

 form a sort of network extending throughout the central part of the lobe. (See also Pis. sxi- 

 XXIV, made from Dr. Gray's excellent photographs.) 



As already stated, the space directly under the lateral-eye lobes is occupied with the ruffle- 

 like masses of the mushroom bodies passing up from the middle on each side of the brain (PI. I, 

 Figs. 2, 5), and branches from the cortical mass of these ruffle-like plates or sheets envelope the 

 lower and outer sides of the lateral optic lobes, until, as represented in PI. I, Fig. 5, the lobes have 

 disapjieared and their place is taken by the chromatic cell layers. Layers of chromatic cells also 

 appear in front between the lower faces of the lateral-eye lobes, as in PI. VII, Fig. 2. The lower 

 tace of the lobes are thus seen to be partly enveloped by layers of chromatic cells. 



The question now arises, have these layers of chromatic gangliou-cells forming in part the cor- 

 tex of the lateral-eye lobes any homologueg in the bi'ain of Arachnida? Without having any sec- 

 tions of the brain of spiders with which to compare them, I am disposed to regard them as pos- 

 sibly identical with the lame glomertilte of the interior eye-lobe of Lycosa, etc., described by Saint 

 Remy. (See his PI. VII, Figs. 72-76, and PI. V III, Figs. 86-88, /. gl.) But until direct comparison 

 be made between these structures in actual sections, it will be difficult to decide this point. Judging 

 by Saint Reuiy's figures alone, they appear to be similar, but I have been unable to find the glom- 

 erules and nervous tubes described by him. At present the chromatic-ceU layer of the lobes 

 under consideration appear to me to be simpler in structure and less difterentiated than in Arach- 

 nida. I have also been unable to detect any bodies homologous with the "medullary plates" of 

 Saint Remy, and which form so characteristic a feature of the upper and lower optic lobes of the 

 brain of Arachnids. In apparently lacking these highly specialized structures the brain of Limu- 

 lus appears to be more primitive. 



* This portion of the lobe is the lame ganglionnaire (Ig) of Viallanes, who divides the optic ganglion into three 

 masses, the most anterior being the "ganglionic plate or sheet, (lame). This sheet or mass is connected with the 

 second mass (me of Viallanes' plate 10, Fig. 17)"'by a bundle of chiasmic fibers or "external chiasma" (che). The 

 third mass (mi) is " united with the cerebral lobe (Ic) by a short peduncle like the optic tract of Crustacea and 

 Insects" (p. 415). These two posterior masses are represented in my PI. II, Figs. 8, 9, and 10, I. eye. I' and I. eye. P; 

 PI. VII, Fig. 2. I had regarded them as connected rather with the cerebral lobes than with the optic lobes, but now 

 accept Viallanes view as to their true relationship. 



t This layer is figured by Viallanes in his PI. 10, Fig. 77, but not described. PI. VII, Fig. 2a. e. eh. represents the 

 " external chiasma" (aic) discovered and so named \tj Viallanes and fairly well seen in my sections. 



