106 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. 



incomplete separation of tin- cells during tin- processes of segmenta- 

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

 next view to be mentioned thai of Ilensen. 



(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 J^ g papers fFi " 59) 

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



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 th- present time. Some of these facts will now he touched upon. 



DKVKLOl'MKM OF 1'KIM I'll KKAL NERVE-TRUNKS IN LEPIDOS1REN 



It ifi Obvious enough from the diversity of statements by skilled 



that the investigation of the method of development nf the 



peripheral m-i'M-s in Vert ebrales is beset, hy technical ditlieullies and 



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



In choose for investigation types of animal in which this liability 



Do error u reduced i> its narrow. -si limits. Such an animal should 



