AND EMBRYOLOGY OF LIMULUS. 31 



2. Smaller ganglion-cells, much more numerous than the larger, more hyaline, having 

 much fewer granules and with the nuclei less distinctly outlined (plate 7, fig. Ic). They 

 are seen to be somewhat smaller, but otherwise like those in the brain of the lobster, 

 which we also figure (plate 7, fig. Id). 



3. Nerve fibres ; these, like the large sized ganglion-cells from which they originate, 

 are stained tawny yellowish brown with osmic acid. These fibres (plate 7, figs. 3a, 3b) 

 are large and coarse, their fine granular contents homogeneous, and they closely resemble 

 the nerve-fibres distributed to the compound and simple eyes of Limulus. Certain fibres 

 near the origin of the optic nerves are distinctly nucleated at intervals (plate 7, fig. 36). 



4. Minute cells, or rather nuclei, very numerous and forming the large ruffle-like 

 masses enveloped in connective tissue and constituting the greater pai't of the brain. 

 They stain dark brown with osmic acid, so that these fungoid or ruffle-like bodies are read- 

 ily distinguishable by their dark brown color from the surrounding tissues, which stain 

 much lighter. In unstained sections simply hardened by alcohol, the tissue or bodies 

 formed by these nuclei is darker than the other tissue, which is white. As these masses 

 or bodies appear to be wholly made up of nuclei, I propose that they be distinguished by 

 the name of nucleogenous bodies. 



The brain itself is enveloped by a very thick, dense membrane, which I am disposed 

 to regard as a neurilemma, homologous with that of the lobster's brain, though much 

 thicker. It is formed of a fibrous connective tissue, and probably some elastic tissue, 

 which directly pienetrates into the brain-substance, forming a network of connective 

 tissue enclosing the nucleogenous bodies ; with occasionally clear nucleated portions in the 

 spaces between the balls of minute nuclei, i.e., the nucleogenous bodies. The fact that 

 this envelop of the brain, a direct continuation of the so-called arterial coat of the oeso- 

 phageal ring, is so intimately connected with the brain-substance itself, and that there is no 

 space between it and the brain for the passage of the blood, and that an artery is situated 

 outside of the brain (plate 7, fig. ^,ar), indicates strongly that this corresponds to the neuri- 

 lemma of other invertebrates, or what Krieger designates as the " perineurium." It forms 

 a fold on the upper side of the oesophageal ring, and thus becomes the direct continuation 

 of the large lateral aortic branches ; but it seems to be formed of short, tortuous fibres of 

 connective tissue, with no true muscular fibres, such as are seen in transverse sections 

 of the smaller arteries. 



We will now describe the topography of the brain as seen in sections, beginning with 

 the upper surface, at the origin of the optic nerves, and going downwards. After the 

 microtome has made five slices y qVo ~ ^o o^ inoh thick, removing the upper part of the low 

 elevations on each side of the broad, shallow median furrow, a section (plate 7, fig. 1) is 

 obtained, which extends through the optic nerves, and also includes a part of the commis- 

 sures by which the brain is connected with the oesophageal ring, the commissures being 

 situated on the dorsal side of the central nervous system ; while the brain is here rather 

 short antero-posteriorly, the median cleft or anterior end of the oesophageal ojDening 

 projecting well into the brain; the latter is more symmetrical in life than indicated in the 

 figure, the brain being probably contracted a little in alcohol, and in the gum, while the 

 razor made a slanting cut, so that while it passed through the middle of the right optic 

 nerve, it merely grazed the edge of the opposite nerve. 



