. Text-book of embryology. Embryology. 204 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. A point to be noticed, of much morphological interest, is that the inner wall myoblasts of Lepidosiren are for a time (Fig. 110, A) in the form of typical myoepithelial cells such as are familiar in some of the lowest invertebrates. They are, as indicated in Chap. II., in continuity with the central nervous system by a protoplasmic tail-like extension of the cell-body closely resembling that which occurs in Nematode worms (Fig. 112). The peripheral portion of this remains as a mass of granular protoplasm on the
. Text-book of embryology. Embryology. 204 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. A point to be noticed, of much morphological interest, is that the inner wall myoblasts of Lepidosiren are for a time (Fig. 110, A) in the form of typical myoepithelial cells such as are familiar in some of the lowest invertebrates. They are, as indicated in Chap. II., in continuity with the central nervous system by a protoplasmic tail-like extension of the cell-body closely resembling that which occurs in Nematode worms (Fig. 112). The peripheral portion of this remains as a mass of granular protoplasm on the surface of the muscle-fibreâthe motor end-plate. The latter is therefore to be regarded as a portion of the muscle-cell which retains its proto- plasmic condition rather than as a portion of the nerve-fibre. The mode of conversion of the embryonic myotome into the muscle-segment has been described as it occurs in Lepidosiren because of the two special safeguards against error which exist in that animal, (1) the large size of the histological units and (2) the fact that the boundary between outer and inner walls of the myotomes is marked by a clear and unmistakable land- mark in the form of the vacuolar zone constituted by the outer portions of the inner wall myo- blasts. It now remains to indicate shortly the more important differ- ences in detail which are to be found in descriptions of the process as observed in others of the lower Vertebrates. The chief of these concerns the fate of the outer wall of the embryonic myotome. In Lepido- siren as has been stated the outer wall gives rise to muscle. In the case of Elasmobranchs and Ganoids, Balfour stated explicitly that the outer wall of the myotome similarly takes part in the development of muscle. Many authorities (Hertwig, Rabl, Maurer), however, deny that this is the case: according to them the outer wall plays no part in muscle-formation : it simply breaks up into amoeboid cells which contribute to the dermal mesenchyme. H
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