n ' NEKVE DEVELOPMENT 107 



be, on the one hand, comparatively archaic — it should belong to one 

 of the relatively more primitive groups of Vertebrates— and, on the 

 other hand, its histological texture should be as coarse as possible, 

 its cell elements being of large size. 



Amongst Vertebrates investigated up to the present time in 

 regard to nerve -development Lepidosiren (Graham Kerr, 1904) is 

 unrivalled in its combination of these qualifications and a summary 

 will now be given of the main features which 1 ; have been made out 

 from the study of the development of the motor nerves in this 

 animal. It will be convenient to commence with the fully formed 

 nerve-trunk and then work backwards towards the earlier and more 

 obscure stages. 



Fig. 61 represents a portion of nerve-trunk from a fully developed 

 larva of stage 34. The nerve-trunk consists of a cylindrical bundle 

 of nerve-fibrils, dotted over the surface of which are the numerous 

 large nuclei of the protoplasmic sheath. The sheath itself is so thin 

 as to be practically invisible even under a high-power immersion 

 objective except in the angle close to a nucleus where it is distinctly 

 visible. 



Fig. 60, D is taken from a larva ten days after hatching. At this 

 stage the nerve-trunk, when examined superficially, has the appear- 

 ance of a thick strand of protoplasm containing numerous nuclei 

 or -a chain of cells. Careful examination of well-fixed and well- 

 t stained specimens shows however that this conspicuous mass of 

 protoplasm is really only the sheath, the true nerve-trunk (w) being 

 visible traversing it from end to end. Scattered about in the thick 

 sheath of this stage there are still to be seen granules of yolk (black 

 in the figure) which have not yet been used up. 



Fig. 60, C is taken from a larva at the time of hatching. At this 

 stage the nerve-trunk is a well-developed bundle of nerve -fibrils, 

 just as in the later stages, but throughout the greater part of its 

 length it is devoid of a sheath of protoplasm. In the section figured 

 the sheath is visible as a mass of nucleated and heavily yolked 

 protoplasm enclosing a portion of the nerve-trunk towards its outer 

 end. This mass of protoplasm is obviously just a condensed part of 

 the general mesenchyme which is scattered about in the form of 

 irregular heavily yolked masses throughout the spaces between the 

 main organ systems. 



The mass in question is identical with the rest of this mesenchyme 

 in its various features and every here and there it is continued into 

 it without a break. The section figured shows the whole length of 

 the motor nerve-trunk from the ventral root to the myoblasts or 

 muscle cells which form the myotome. Towards its outer end the 

 trunk is seen to break up into numerous diverging strands which 

 are directly continuous with the protoplasm of the myoblasts (see 

 below, p. 204). 



Fig. 60, B is taken from an embryo about three days before 

 hatching. At this stage the myotome has barely commenced to 



