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-fibre — the 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. Hence 

 these investigators term the outer wall of the myotome the 

 " Cutis-layer." In the case of the Sturgeon, Maurer corroborates 

 Balfour's statement that the myotome is composed of two layers of 

 muscle-elements but according to him the outer layer is simply 

 budded off from the inner and does not represent the original 

 outer wall of the myotome as Balfour supposed. 



In the Amniota the myotome in early stages is almost square as 

 seen in a transverse section practically the whole of the wall 



FIG. 112. — Diagram of a motor ganglion- 

 cell in the spinal cord continuous 

 through the substance of a nerve-fibre 

 ■with a muscle-cell in the myotome. 



c./, contractile fibrils in myoepithelial cell ; 

 m, myoepithelial cell ; n.c, ganglion - cell ; n.f, 

 motor nerve-fibre. 



