102 EMBRYOLOGY OF THK I.nWKl; \ KLTEBRATES en. 



They are sometimes very distinct in the hind-brain of fowl em- 

 bryos of about the fourth day (see Fii:- -"><' in chap. X.). It is 

 however an outstanding characteristic of tin* head region as compared 

 with the trunk that the segmentation of its mesoderm lias become 

 Idurred and in great part obliterated. It is under those ciroumstancefl 

 improbable that a primitive segmentation of the central nervous 

 system, which is in its nature linked up to a segment a thm of 

 mesodermal structures, should have remained particularly distinct 

 in a region where the Diesodermal segmentation itself has become 

 particularly obscure. 



The appearances mentioned seem to be adequately explained by 

 the active growth of the developing brain within its confined space, 

 combined with the presence round it of mesodermal tissue with 

 vestigial segmentation. It will be noticed in the figure referred to 

 above that the dividing lines between the " neuromeres " are spaced 

 out at exactly the levels where we should expect to see boundaries 

 of mesoderm segments were the existing series prolonged forwards. 

 Segments are no longer visible in this region but there is, as will 

 appear later, convincing evidence that the series of segments 

 did formerly extend through this region now occupied by continuous 

 mesenchyme. 



It may well be that the individuality of the segments, no longer 

 visible as such, is still expressed by a difference in consistency of 

 the mesenchyme, sufficient to mould by its varying resistance the 

 actively growing hind-brain as it presses against it. 



DEVELOPMENT OF THE PERIPHERAL NERVES. The development 

 of the peripheral nerves of Vertebrates has been the subject of a 

 large amount of investigation, partly on account of its intrinsic 

 interest and partly on account of its bearing upon physiology and 

 pathology. In spite of the labours of numerous investigators tl it- 

 problem for we may take it that the mode of development is //'?&' /"- 

 mentally the same throughout has not yet by any means been 

 satisfactorily decided. 



While bearing in mind the undesirability of making use of 

 modern facts merely to support, or to undermine, old hypotheses, it 

 will l>e convenient to approach the question by stating shortly the 

 t hive prevalent views as to the main features of the development- 

 of peripheral nerve-trunks neglecting differences in detail. Kor 

 shortness these three views may be termed alter their nmst 

 prominent supporters (1) the His view, (2) the Balfour view and 

 (3) the Hensen view. 



THK His VIKW (Outgrowth theory). This hypothesis may In- said 

 to have been found. -d by KupfVer in the rmbry<>lLrieal portion of 

 Hiddcr and Kuplfer's work (lSf>7 n the spinal cord. As however 

 KupH'.-r latT nn L Mve up the view, in favour of that of Uall'mir. t he 

 bvpOtheflU now under Consideration is eumniouly associated with the 

 name of His. who pl.m-d the main part in building up i he theory 

 and who full its principal founder. 



