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. 



FRORIEP 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 



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

 BALFOUB. 



li>>, Hind-brain; vg, vagus; ep, epiblast ; ch, chorda; x, thickening of hypoblast (possibly a 

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

 mesoblast ; */, splanchnic mesoblast (Durmseitenplaite) ; hy, hypoblast. 



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

 nerves can be assigned to separate segments in muck the same nw / 

 as in the trunk. In this particular the conditions are clean >t 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 Wu i , 



According to WIJIIE nine * segments are distinguishable in the 

 of Selachians. To the iii>t .-cgment Ix-lnn^s tl;- mums 



* fBccent investigations indicate that the head-segments in Selachians are 

 'i more numerous. TRANSLATOR.] 



