454 EMBRYOLOGY. 



suitable objects together with stainicg of the nerve-fibrillse, isolation 

 of the elements preceded by maceration and staining) must be 

 employed. 



Having thus sketched out the various standpoints taken by numer- 

 ous investigators on the question of the source of the peripheral 

 nervous system, I give a number of observations that have been 

 made tipon the development of certain nerves. These relate to the 

 development of : 



(1) The ventral and dorsal roots of the nerves ; 



(2) Certain large peripheral nerve-trunks, as the nervus lateralis ; 



and 



(3) The nerves of the head and their relation to the spinal nerves. 



(I) Of the roots of the nerves the anterior [ventral] are de- 

 monstrable earlier. There may be distinguished three stages in 

 their development. 



The first stage has been observed by DOHRN and VAN WIJHE in 

 Selachian embryos. At a time when the neural tube has not yet 

 developed any mantle of nervous substance, and the muscle-segment 

 still lies very close to it, there arises between the two a connection in 

 the form of a very short protoplasmic cord. The fundament of the 

 nerve is therefore, as VAN WIJHE remarks, ab origine near its 

 muscle-complex, from which it never separates. Soon after this it 

 is elongated by the removal of the muscle-segment farther from the 

 neural tube ; it increases in thickness and now encloses numerous 

 nuclei, and possesses therefore a cellular composition, a condition 

 which I shall designate as second stage. 



There is a difference of opinion as to the cells which make their 

 appearance in the fundament of the nerve. Whereas KOLLIKER 

 His, and SAGEMEHL recognise in them immigrated connective-tissue 

 elements, which are destined to form simply the envelopes of the 

 nerves, BALFOUR, MARSHALL, VAN WIJHE, DOHRN, and BEARD main- 

 tain that they migrate out from the spinal cord and share in the 

 development of the nerves themselves. BEARD even derives the 

 motor terminal plate from them. Soon after, as is asserted, 

 connective-tissue cells from the surrounding mesenchyme become 

 associated with the nerve-cells derived from the spinal cord and 

 ordinarily become indistinguishable from them. 



Finally, in the third stage the cellular fundament of the motor 

 root acquires a fibrillar condition (fig. 260 vw), and it now becomes 

 possible to trace the origin of the nerve-fibrillse in the spinal cord 

 from groups of embryonal gangl ionic cells or neuroblasts (His). 



