[134] Comp. Problem I. § [130], with the remarks of Charles Robin, Anatomie et Physiologie Cellulaires, 1873, p. 20.
[135] Kleinenberg, Hydra; Eine Anatomisch-Entwickelungs-Untersuchung, 1872, p. 11. Eimer, Zoologische Studien auf Capri, 1873, p. 66.
A similar formation is described by Dr. Allman in the Myriothela; he says, however, that he has never been able to trace a direct continuity of the caudal processes of the cells with muscular fibrils. He believes that the processes make their way to the muscular layer through undifferentiated protoplasm.—Philos. Transactions, Vol. CLXV. Part II. p. 554.
An intermediate stage between this neuro-muscular tissue and the two differentiated tissues seems presented in the Nematoid worms which have muscles that send off processes into which the nerves pass. Gegenbaur declares his inability to decide whether these processes are muscles or nerves. Bütschli thinks the nerve-process blends with the muscle-process.—Archiv für mikros. Anatomie, 1873, p. 89.
[136] “The gray matter of the cord seems undoubtedly to be formed by a metamorphosis of the external cells of the epiblast of the neural tube, and is directly continuous with the epithelium; there being no strong line of demarcation between them.”—Op. cit., p. 185.
[137] Robin, Anat. et Physiol. Cellulaires, p. 332.
[138] Stilling, Bau der Nervenprimitiv-Fasern, 1856, p. 16.
[139] “There was a time,” says Kölliker, “when I confidently believed that an hypothetical explanation of the arrangement of elements in the spinal cord could be grounded on a basis of fact; but the deeper my insight into the minute anatomy, the less my confidence became; and now I am persuaded that the time is not yet come to frame such an hypothesis.”—Gewebelehre, 5te Auf. 1867.
[140] In the Gasteropoda the cells range from 220 μ to 3 μ (μ = 0,001 millimètre).
[141] Haeckel, Müller’s Archiv, 1857. Leydig, Vom Bau des thierischen Körpers, 1864, I. 84. Robin, Anat. et Physiol. Cellulaires, p. 89. Should the observations of Heitzmann be confirmed, there would be ground for believing that neurine is normally fibrillated. He says that the living protoplasm in the Amœba, white blood-corpuscle, etc., is an excessively fine network, which condenses into granules at each contraction. (Cited in the Jahresberichte über Anat. und Physiol., 1873, Bd. II.) Walther, who examined frozen brains, describes the cells as quite transparent at first, with very rare granules, but gradually while under observation the granules became more numerous. Centralblatt, 1868, p. 459. According to Mauthner, Beiträge zur Kenntniss der morphologischen Elemente des Nervensystems, 1862, p. 41, neurine has three cardinal forms—transparent, finely granular, and coarsely granular.