384 JOHN BEARD, 



cells. Among a hundred embryos examined, there are perhaps not 

 more than twenty-five in which the nerves of the transient system 

 exhibit a very striking grade of development. When such is the case, 

 there are always many of them in a single embryo. They often, but 

 by no means invariably, occur in pairs one on each side of the body 

 (Figs. 24, 75 etc.), or at least so nearly opposite that a conclusion 

 as to their frequently paired character seems to find a warrant. No 

 more than any other parts of the transient system do 

 these nerves manifest traces of segmental arrangement 

 — a fact doubtless of import in any considerations as to their true 

 nature. 



As already described, the course of these nerves is always directed 

 towards the epiblast, so that ultimately they extend immediately beneath 

 it. Usually , in order to get to this position , they arch over the 

 myotome (Fig. 75), but, in later stages especially, they may pierce the 

 apex of this structure on their way (Figs. 62, 6o). 



It is a little doubtful whether the final termination of any, even 

 of the longest, of these nerves has yet been determined '). A free 

 mode of ending, be the course long or short , is the only condition 

 hitherto observed. 



Often, as in Figs. 23 and 72, the nerves extend the whole length 

 of the myotome, proceeding onwards past the segmental duct, and 

 then for some distance in the direction of the yolk-sac between epi- 

 blast and somatopleure. The above figures prove that they almost, 

 if not quite, reach the yolk-sac. 



No pains have been spared in an attempt to demonstrate such a 

 termination, but for the moment it is rather a surmise, based on 

 great probabilities, than a proved fact. 



No one has, as yet, announced the discovery of nerves of any 

 kind upon the yolk-sac, and there are doubtless many embryologists 

 who feel so certain of the existence of such nerves that they regard 

 a demonstration of it as unnecessary. 



However, if a fact can be demonstrated , every eifort must be 

 made to substantiate it. Failing conclusive proof probabilities must 



be relied on. 



In a few preparations of the yolk-sac covering of embryos of 

 14 — 25 mm no traces of nerve-fibres on that structure were encountered. 



1) Neither the "Goloi" nor the Ehrlich methyl-blue method has 

 given useful results in Uaja. 



