192 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



semi-fluid, homogeueous substance, which is the first to take up the 

 methyien-blue staiu. It has been called by Bethe ('98) the "peri- 

 fibrillar substance." The accumulation of this fluid into drops gives 

 the characteristic beaded appearance of methylen-blue preparations. 



A distinct nucleated myelin sheath surrounds both the fibre and the 

 peripheral ganglion cells of Palaemonetes. This sheath, which stains 

 intensely black in Vom Rath's platino-osmic fixative, can be traced 

 some distance beyond the peripheral ganglion cells toward the sensory 

 hairs, and also centrally into the brain, where it ceases only when 

 the fibres enter the neuropil substance. Figure 16 (Plate 4) shows 

 a ganglion cell and its peripheral process surrounded by the sheath. 

 Elongated, flattened nuclei occur at intervals along the walls of the 

 sheath, curved around it and the enclosed fibre ; certain of these 

 sheath nuclei can be seen in Figure 4 (jil. tu.) between the ganglionic 

 cells and the brain, though the myelin sheaths are not stained in this 

 haematoxylin preparation. Quite frequently one of them may occur 

 in close proximity to a ganglion cell. Thus are produced (Plate 4, 

 Fig. 17) appearances which might be mistaken for a ganglion cell with 

 two nuclei. Careful study, however, shows that one nucleus {nl.) lies 

 within the cell, the other {nl. tu.) without, but abutting on the 

 ganglion cell so closely as to somatimes change its form. In every 

 instance of this kind one of the nuclei, owing to its irregular outline, 

 its smaller size, and the curved form which it takes in adaptation to 

 the surface of the cell, could be identified as belonging to the sheath 

 rather than to the nerve cell. 



The peripheral ganglion cells are much elongated and are of the 

 typical bipolar form (Plate 4, Fig. 18). They measure from 10 to 

 14 /x in diameter; their nuclei are relatively large, measuring from 

 7 to 9 /A in diameter, and are usually ovate in outline, their length in 

 some cases being twice as great as their diameter. One large spherical 

 nucleolus is usually present in the chromatic network, though some- 

 times two or more are found. No definite structure can be recognized 

 in the cytoplasm of the cell, nor any traces of fibrillaj ; this, however, 

 is not strange, as the cell usually stains so intensely that it would not 

 be reasonable to expect to make out its finer structure. In methylen- 

 blue preparations a narrow zone about the nucleus stains only faintly, 

 the coloration becoming more intense as the periphery of the cell is 

 approached ; so here, as Bethe also found in the nerve cells of Carcinas, 

 the chromatin granules are more numerous at the periphery of the cell 

 cytoplasm, and nearly wanting around the nucleus. 



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