In this connection should be considered the careful work of Meigs (is) (1908) on the wing-muscle of the fly. He recognizes the existence of at most only 3 different substances in the sarcostyle, that of the Q bands and that of the Z and M lines. He regards J as the optical effect of reflection phenomena due to the presence of Z. He conceives of Z as a substance more highly refractive than its surrounding medium Q. His identification of M as a true membrane in the wing-muscle of the fly is at variance with the observations on the wing-muscle of beetles by Thulin (4) who records its absence in this group. In Limulus striped muscle, both cardiac and skeletal, M-membranes can not be discerned either in fresh or fixed and stained material.
In unstained skeletal fibers the Q-band is faintly visible; in cardiac muscle it can only occasionally be barely detected. But in stained fibers in certain instances it is distinctly visible in both types of muscle (figs. 6B and 9). Certainly Q and J have a different staining capacity; and this indicates a structural difference, perhaps inhering only in a relatively greater abundance of "anisotropic granules" in Q.
The J-band, compared with the Z-membrane, is much too wide to permit of any reasonable interpretation of its appearance in terms of refraction phenomena due to the presence of the Z-membrane. More over, in the contracted fiber one sees only "contraction bands" (appar ently Z Q), and J-bands, the J-band being now topographically in part at the former level of the original Q-band. In the case of Limulus striped muscle, one is forced to conclude that 3 distinct substances are present, namely, that of the telophragmata and the J and Q substances.
Limulus increases in size at least throughout early life. Histologi cally the tissues of the internal organs mature early, but provision must be made for constant enlargement. This fact must be kept in mind in the interpretation of the structure of its muscle. Though mature as concerns its fundamental histogenesis, it presents develop mental phenomena. It is of cardinal interest and significance that these are very similar to early histogenetic stages in the muscle of higher forms. In skeletal muscle this point concerns chiefly the arrange ment of the fibrillae in lamellae and cylinders, each undergoing longi tudinal splitting, the former both radial and vertical (paratangential).
This is exactly the condition prevalent in developing muscles of the newly-hatched rainbow trout and other teleosts. In cardiac muscle this same point concerns also the nucleus. These nuclei multiply greatly, and by amitotic division. The reason for amitotic multipli cation rather than mitotic in the enlarging Limulus muscle, both skeletal and cardiac, remains obscure.
This leads naturally again to a consideration of the evidence for "muscle cells" in the sense of Apathy (1888) and Baldwin (1912).
The original myoblasts are lost in a syncytium, in the skeletal muscle appearing as multi-nucleated distinct fibers, in the cardiac muscle as a loose-meshed intricate network of larger and smaller trabeculle. The muscle-substance here consists of a finely granular sarcoplasm (with large lipoid spherules in the vicinity of the nucleus), throughout which are scattered the myofibrils and the nuclei. The relationship of ground membrane to nuclear wall and to sarcolemma, as also the result of the abundant nuclear amitosis in cardiac muscle, would hardly be intelli gible on any other ground. In view of the evidence derived from the study of the muscle from the adult mouse (Jordan), and the data recorded by Asai (3) on the histogenesis of this same heart-muscle, the cellular hypothesis of muscle structure must be definitely abandoned.
If additional evidence were required in refutation of the interpreta tion of interfibrillar muscle-cells in striped muscle, it may be drawn from the structure of the striped muscles of tunicates (e. g., tail-muscles of Amaroucium); here the "fiber" is a mononucleated elongate hexagonal cell, with its nucleus centrally and the myofibrilke peripherally disposed. - The observations here recorded with respect to the fibrillar structure of muscle, namely, the resolution of the muscular substance into finer and finer complexes to the limit of visibility, strongly sup port the protomere hypothesis of Heidenhain (s), which maintains that all living matter is divisible into progressively smaller specific units, the ultimate vital molecular units being the "histomeres," "proto meres," or "metafibrillie."