Presidential Address. 11 



form formidable barriers in the way of progress when they become elevated 

 in the worker's mind to the level of scientific theory. 



II. The Development of the Peripheral Nerves of Vertebrates. 



The problem of the mode of development of the peripheral nerves of 

 the Vertebrate is one of great interest and importance, not merely to the 

 embryologist, but also to the morphologist, the physiologist, and the 

 pathologist. It has for long been the subject of controversy, and quite 

 recently it has attracted much attention owing to the appearance of a 

 number of important publications upon the subject, amongst which I need 

 only mention those of Harrison 1 and Held. 2 



In making the endeavour to arrive at a true solution of this or any 

 similar problem of histogenesis, an all important preliminary is to select 

 the most favourable available material for investigation. To choose un- 



o 



suitable material is to run the risks of becoming lost amidst the details of 

 observations, of being led astray, and of failing completely to reach the goal. 

 I think that most morphologists will admit that there are two chief criteria 

 to be employed in deciding upon suitable material. 



(1) The animals should belong to the lower or more primitive groups 

 of the gnathostomes, for upon the whole an animal which retains primitive 

 characters in the adult state is more likely to retain relatively primitive 

 characters in the earlier stages of development. In other words, the mode 

 of development of its various organs is, on the whole, less likely to be 

 obscured by secondary modifications. 



(2) The animal selected should be one with a histological texture as coarse 

 as possible ; the cell elements composing its tissues should be of large size. 



The groups of the G-nathostomata which are generally regarded as the 

 most primitive are the Elasmobranchs, the Crossopterygians, and the Lung- 

 fishes, and amongst these there is one, and so far as is known only one, 

 form — Lepidosiren — which is characterised by the remarkable coarseness 

 of its histological texture, and by the great size of its cell elements. On 

 this account it must be admitted that Lepidosiren is an extraordinary 

 favourable object upon which to tackle any such problem as that under 

 consideration. I therefore feel justified in accentuating the points made 

 by the study of its development. 3 



1 Harrison, '' Embryonic Transplantation and Development of the Nervous System," 

 Anat. Record, ii., Dec. 1908. 



2 Held, Die Enhvicklung des Nervengewebes bei den Wirheltieren, Leipzig, 1908. 



3 C.f. Grabam Kerr, Trans. Roy. Soc. Edin., xli., 1904, p. 119 ; and Proc. Roy. Physical 

 Soc. Edin., xvi., 1906, p. 206. 



