Fig. 376. Muscle-cells of Lizzia Köllikeri. (From Lankester; after O. and R. Hertwig.)
A. Muscle-cell from the circular fibres of the subumbrella.
B. Myoepithelial cells from the base of a tentacle.
The number of the fibrils of each cell gradually increases, and the protoplasm diminishes, so that eventually only the nucleus, or nuclei resulting from its division, are left. The products of each cell probably give rise, in conjunction with a further division of the nucleus, to a primitive bundle, which, except in Amphioxus, Petromyzon, etc., is surrounded by a special investment of sarcolemma.
The voluntary muscular system. For the purposes of description the muscular system of the Vertebrata may conveniently be divided into two sections, viz. that of the head and that of the trunk. The main part, if not the whole, of the muscular system of the trunk is derived from certain structures, known as the muscle-plates, which take their origin from part of the primitive mesoblastic somites.
Fig. 377. Transverse section through the trunk of an embryo slightly older than fig. 28 E.
nc. neural canal; pr. posterior root of spinal nerve; x. subnotochordal rod; ao. aorta; sc. somatic mesoblast; sp. splanchnic mesoblast; mp. muscle-plate; mp´. portion of muscle-plate converted into muscle; Vr. portion of the vertebral plate which will give rise to the vertebral bodies; al. alimentary tract.
It has already been stated (pp. 292-296) that the mesoblastic somites are derived from the dorsal segmented part of the primitive mesoblastic plates. Since the history of these bodies is presented in its simplest form in Elasmobranchii it will be convenient to commence with this group. Each somite is composed of two layers—a somatic and a splanchnic—both formed of a single row of columnar cells. Between these two layers is a cavity, which is at first directly continuous with the general body cavity, of which indeed it merely forms a specialised part ([fig. 377]). Before long the cavity becomes however completely constricted off from the permanent body cavity.
Very early ([fig. 377]) the inner or splanchnic wall of the somites loses its simple constitution, owing to the middle part of it undergoing peculiar changes. The meaning of the changes is at once shewn by longitudinal horizontal sections, which prove ([fig. 378]) that the cells in this situation (mp´) have become extended in a longitudinal direction, and, in fact, form typical spindle-shaped embryonic muscle-cells, each with a large nucleus. Every muscle-cell extends for the whole length of a somite. The inner layer of each somite, immediately within the muscle-band just described, begins to proliferate, and produce a mass of cells, placed between the muscles and the notochord (Vr). These cells form the commencing vertebral bodies, and have at first ([fig. 378]) the same segmentation as the somites from which they sprang.
After the separation of the vertebral bodies from the somites the remaining parts of the somites may be called muscle-plates; since they become directly converted into the whole voluntary muscular system of the trunk ([fig. 379], mp).
According to the statements of Bambeke and Götte, the Amphibians present some noticeable peculiarities in the development of their muscular system, in that such distinct muscle-plates as those of other vertebrate types are not developed. Each side-plate of mesoblast is divided into a somatic and a splanchnic layer, continuous throughout the vertebral and parietal portions of the plate. The vertebral portions (somites) of the plates soon become separated from the parietal, and form independent masses of cells constituted of two layers, which were originally continuous with the somatic and splanchnic layers of the parietal plates ([fig. 79]). The outer or somatic layer of the vertebral plates is formed of a single row of cells, but the inner or splanchnic layer is made up of a kernel of cells on the side of the somatic layer and an inner layer. The kernel of the splanchnic layer and the outer or somatic layer together correspond to a muscle-plate of other Vertebrata, and exhibit a similar segmentation.