454 KMBRYOLOGY. 



suitable objects together with staining of the nerve-fibrillae, 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 upon 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. 



(1) 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 Dohen 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 

 stUl 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 difierence of opinion as to the cells which make their 

 uppearance in the fundament of the nerve. Whereas Kolliker 

 His, and Sagemehl recognise in them immigrated connective-tissue 

 elements, which are destiaed 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 ganglionic cells or neuroblasts (His)« 



