Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/157

Rh embryos; but the deficiency in my researches may be supplied from the description given by Valentin (Entwicklungs-Geschichte, p. 268), from which the following passage is extracted: “Long before separate muscular fibres can be discerned, the globules of the primitive mass are seen, arranged in parallel lines, particularly when they are lightly pressed between two pieces of glass. The granules then appear to be drawn somewhat nearer together, to become completely coalesced, in some situations, while at others the blending takes place only on the one or the other side, and to be combined into one transparent mass. In this way filaments are formed, which, in some situations, have an appearance like strings of pearls, at others, on the contrary, are less sharply indented; they often also continue slightly puckered on one side, whilst the margin of the other has already become more straight. At a subsequent period, all trace of granules or division in the filament vanishes, and its outline becomes symmetrically transparent and cylindrical. The muscular fibre usually undergoes no other change until the sixth month, except that its substance becomes somewhat darker and its cohesion closer. The first traces of transverse striæ are exhibited in the sixth month. These fibres are the primitive fasciculi of muscle and not the primitive fibrils, which latter are formed by a splitting of the fasciculus into smaller fibres. From the period at which the muscular filaments become transparent and uniform, masses of globules, of a more or less spherical form and somewhat larger than the blood-corpuscles, begin to accumulate between them. They diminish again afterwards, and, blending with the gelatiniform mass which connects them, become converted into the connecting areolar tissue.”

The youngest embryos in which I have investigated the generation of muscle were those of the pig, measuring three and a half inches in length. If a portion of one of the superficial dorsal muscles be removed from an embryo pig of that size, and examined under the microscope upon a black ground, a transparent gelatiniform mass is observed, in which parallel fibres (primitive fasciculi of muscle) run in close contact, having a whiter appearance than the surrounding gelatinous substance. As development proceeds, the transparent sub-