Presidential Address. 17 



found that with proper precautions he could keep these fragments living 

 in clotted frog's lymph, on a hollow slide under a coverslip, for a period 

 of weeks on end. Now when fragments of embryonic spinal cord were 

 observed in this way, it was found that after a day or two of cultivation 

 a considerable number of nerve rudiments were seen extending out from 

 the piece of spinal cord into the clotted lymph in whicli it was embedded. 

 The free end of the nerve rudiment projected into irregular tags of proto- 

 plasm, which showed active amoeboid movements. Measurements of 

 individual rudiments at short intervals showed marked increase in length — 

 as much as 20 fi in twenty-five minutes. These are the essential 

 observations upon which Harrison's conclusions are based. 



Now the question at once suggests itself: Has Harrison excluded the 

 possibility that his excised fragments of embryonic spinal cord included 

 the short nerve trunk rudiments, like those figured in my Figs. 1 and 2 ? 

 He naturally has not. Such rudiments have only been observed in sections 

 of particularly favourable material, fixed and stained with great care, and 

 their detection amongst a mass of heavily yolked living cells would be 

 a practical impossibility. Granting that such nerve rudiments were present 

 in the excised fragments, as in all probability they were, what do the 

 experiments prove ? Simply that the young nerve grows in length — which 

 is, of course, a self-evident fact, quite independent of any particular theory. 

 For the demonstration of the truth of His' theory, what is needed is not 

 a proof of this well-known fact, but a demonstration that the young nerve 

 shows a differential rate of growth greater than that of the tissues amongst 

 which it is normally situated, so that its free end moves through or between 

 these tissues until it reaches its definite end organ. 



These short remarks are sufficient to indicate that, in my opinion, 

 Harrison's experiments, beautiful and interesting as they are, do not by 

 any means necessarily have the finality that has been attributed to them. 1 



Myoejnthelial Cells in Vertebrates. 



There is one other point in connection with the developing motor nerve 

 trunks of Lepidosiren which I wish to accentuate, viz., that it is occasionally 

 possible to demonstrate the fact that there is absolute continuity of substance 

 between the nerve trunk and the myoblast or young muscle cell. The young 

 muscle cell is, in fact, a myoepithelial cell with a protoplasmic nerve tail as 



1 Hensen (Physiol. Verein Kiel., 18th Nov. 1907) somewhat caustically points out that 

 Harrison must be thanked for such a striking demonstration that after all embryonic 

 nerves can grow along a wrong path ! 



VOL. XVIII. B 



