382 JOHN BEARD, 



of note that one may, as in Fig. 45, find two similar cells ^), one in 

 each myotome directly opposite each other, or so nearly so as to 

 suggest a bilateral arrangement. Such a find is interesting because 

 it seems to indicate a former greater symmetry and co-ordination, or 

 a more defined murphclogical structure of the apparatus, than at 

 present obtains in Raja, — just as the occasional presence of a few 

 odd transient ganglion -cells and nerve-processes in Mustelus vulgaris 

 is evidence in support of a belief that this form once possessed a 

 transient nervous apparatus of the type of that of Raja. 



The wandering or peripheral transient ganglion-cells are seldom 

 if ever multipolar, usually they are bipolar (Fig. 43) with two nerve- 

 processes, sometimes (Fig. 54) they would seem to be unipolar , and 

 those embedded in the myotome frequently give no evidence of the 

 possession of any processes whatever. This latter observation may 

 not be regarded as an error : it is explicable if one may assume that 

 such cells have lost their inherited functions, and are degenerate 

 without, at first showing, other signs of atrophy. 



The peripheral ganglion-cells resemble the central ones in ap- 

 pearance and size: like the latter they acquire, but in comparatively 

 late stages, an investment of capsule-cells (Fig. 40). 



Finally, it must be mentioned that they frequently form chains, 

 as shown in Figs. 42 and 51, reaching from the centre to, or even 

 over, the tip of the myotome. The destiny of all these cells will be 

 discussed at a later stage. 



c) Relations to spinal ganglion-cells. Many of the transient ganglion- 

 cells often appear to be in intimate association with cells of the spinal 

 ganglia. It is, however, not probable that there is anything but accident in 

 this. Certainly any morphological relationship is quite out of question. 



The close union and association which is recognisable in many 

 of the figures, and which is bound to attract the observer's attention 

 in studying sections of Raja embryos, finds a natural explanation in 

 the circumstance that both sets of cells have a locus of origin im- 

 mediately laterad of the medullary plate. The epiblast on both sides 

 is here the seat of origin of future spinal ganglion-cells on the one 

 hand and of future transient cells on the other. Hence a certain 

 entanglement of no morphological consequence frequently ensues. 



d) Transient nerves of the system. Once more the 

 examination of sections of early embryos, as previously defined, reveals 



1) see page 352. 



