458 EMBRYOLOGY. 



independently of the neural ridge, as individual outgrowths of the- 

 brain-vesicles nearer their base, and are comparable with the ventral 

 roots from the spinal cord. 



FROEIEP finds that the hypoglossus in Mammals possesses dorsal roots, 

 with small ganglionic fundaments, in addition to ventral roots. The latter 

 subsequently undergo degeneration. 



The agreement between cranial and spinal nerves which is ex- 

 pressed in this method of development, becomes still greater and 



Fig. 261. Cross section through the hind part of the head of a Chick embryo of 30 hours, after 

 BALFOUR. 



hl>, Hind-brain; vg, vagus; ep, epiblast ; ch, chorda; x, thickening of hypoblasfc (pDssibly a. 

 rudiment of the subchordal rod) ; al, throat ; ht, heart ; pp, body-cavity ; so, somatic 

 mesoblast ; sf, splanchnic mesoblast (Darmseitenplatte) ; hy, hypoblast. 



acquires a further significance from the fact that in the head also the 

 nerves can be assigned to separate segments in much the same manner 

 as in the trunk. In this particular the conditions are clearest in 

 the Selachians, where, in fact, the head-segments have been most 

 thoroughly investigated, so that 1 limit myself to a statement of the 

 results acquired in this field by WIJHE. 



According to WIJHE nine* segments are distinguishable in the 

 head of Selachians. To the first segment belongs the ramus 



* [Recent investigations indicate that the head-segments in Selachians are 

 much more numerous. TRANSLATOR.] 



