102 EMBEYOLOGY OF THE LOWEE VEETEBEATES CH. 



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

 bryos of about the fourth day (see Fig. 2j36 in Chap. X.)- It is 

 however an outstanding charaeteristic of the head region as compared 

 with the trunk that the segmentation of its mesoderm has become 

 blurred and in great part obliterated. It is under those circumstances 

 improbable that a primitive segmentation of the central nervous 

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

 mesodermal structures, should have remained particularly distinct 

 in a region where the mesodermal 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 the 

 problem — for we may take it that the mode of development is funda- 

 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 be convenient to approach the question by stating shortly the 

 three prevalent views as to the main features of the development 

 of peripheral nerve -trunks neglecting differences in detail. For 

 shortness these three views may be termed after their most 

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

 (3) the Hensen view. 



The His View (Outgrowth theory). — This hypothesis may be said 

 to have been founded by Kupffer in the embryological portion of 

 Bidder and Kupffer's work (1857) on the spinal cord. As however 

 Kupffer later on gave up the view, in favour of that of Balfour, the 

 hypothesis now under consideration is commonly associated with the 

 name of His, who played the main part in building up the theory 

 and who fully deserves to be regarded as its principal founder. 



