106 EMBEYOLOGY OF THE LOWER VERTEBRATES ch. 



incomplete separation of the cells during the processes of segmenta- 

 tion and cell division — the view in his hands came to approach the 

 next view to be mentioned that of Hensen. 



(3) The Hensen View (Primitive continuity theory). — This view 

 has found its strength in general physiological considerations 

 rather than in convincing facts of observation. According to Hensen 

 (1864, 1868, 1903) the nerve which connects centre with end-organ is 

 a primary connexion which has been there from the beginning. It 

 existed first as a simple bridge of protoplasm, such bridges being 

 present between the various cells of the body owing to the fact that 

 the processes of segmentation and cell division are not complete so as 

 to lead to absolute isolation of the cells or segments from one another. 

 According to this view the growth in length of a nerve -trunk is 



simply the extension 

 of a pre - existing 

 bridge, as the organs 

 at its two extremities 

 — centre and end- 

 organ — are pushed 

 apart from ' one an- 

 other^ during the course 

 of development. 



Fig. 59. — Illustrating Hensen's view of the origin of peri- Hensen figures in 



pheral nerves. The section is taken from a 9-day Rabbit ],jo -nfmers (Tiff 59^1 

 embryo, and passes through the trunk region. (After , , , . , , n 



Hensen, 1903.) what he takes to be 



such nerve-rudiments, 

 in the form of numerous fine filamentous structures passing across 

 the space between spinal cord and myotome. There is however 

 no evidence to show that these filaments have anything to do with 

 nerve-trunks. Although for this reason it is impossible to accept 

 the main observational basis of Hensen's view, that does not neces- 

 sarily invalidate the physiological considerations which may be held to 

 give an a priori probability to the correctness of his general theory. 



The three views which have been outlined above were fashioned 

 by their respective authors long ago as embryological science goes. 

 Since then new facts have become known which have to be taken into 

 account when considering their acceptability as working hypotheses 

 at the present time. Some of these facts will now be touched upon. 



DEVELOPMENT OF PERIPHERAL NERVE-TRUNKS IN LEPIDOS1REN 



It is obvious enough from the diversity of statements by skilled 

 observers that the investigation of the method of development of the 

 peripheral nerves in Vertebrates is beset by technical difficulties and 

 resulting liability to error. In such a case it is of special importance 

 to choose for investigation types of animal in which this liability 

 to error is reduced to its narrowest limits. Such an animal should 



