338 JOHN BEARD, 



what might be termed plexiform, in that each appears to be made 

 up of several ganglion-cells and their processes. Of these ganglion- 

 cells some seem to be in contact merely, or applied end to end; but 

 two at least, to the left of Figs. 8 and 9, look as if intercalated in 

 the course of the axis-cylinder. 



These two nerves could not be followed as far ventralwards as 

 some to be described in other embryos, but their extension nearly to 

 the ventral limit of the myotome is apparent from the figures. 



Embryo No. 141 reveals a rich development of such sub-epiblastic 

 nerves. It is worthy of notice that such do not appear in the same 

 relative abundance in every embryo from this stage onwards. In some 

 embryos they are far better represented than in others, even of the 

 same age and general characters : a circumstance which would demand 

 discussion in a later portion of the work. 



Another incident about these sub-epiblastic nerves of the present 

 and some other embryos is that they would appear to occur very 

 frequently in pairs one on each side of the body. This conclusion 

 finds its warrant in the observation, repeatedly made in the present 

 embryo, that, wherever one of these nerves was met with on the one 

 side, a corresponding nerve of the other side was either present in 

 the same section, or followed immediately in the ensuing ones. 



Another series (No. 143) i), in which 86 somites were counted, 

 though slightly "older" in respect of the number of somites and in 

 some other particulars, yields earlier stages of the development of the 

 transient system. In it there are very few ganglion-cells and not 

 many transient nerves developed. As in other cases the transient 

 ganglion-cells here also make their appearance in the sections just 

 directly in front of the pronephros. 



The series shows very clearly the early development of the anterior 

 roots of spinal nerves in the form of cellular chains. The length of 

 the embryo was noted as 10 mm — it is possible that this was a 

 slight error. 



Four sections are figured from this series (Figs. 14, 15, 16 and 

 17). Fig. 14 shows one of the ganglion-cells {sp.c) resting on the 

 apex of the myotome, but connected with the as yet only developing 

 ganglionic centre by means of a fibre (n.p) which it has spun out. 



1) The gill-clefts and pouches show the grade of development of 

 No. 141. 



