264 



THE CHICK. 



between the cerebral vesicles, form commissural bands, which 

 for a time connect together the successive pairs of nerve 

 outgrowths. 



In a typical cranial nerve, such as the facial or glosso-pharyn- 

 geal, the further changes are as follows. The nerve rudiment 

 rapidly extends outwards, lying close beneath the external 

 epiblast, but independent of this. Opposite the nerve, but at 

 some little distance beyond the brain, and about the horizontal 

 level of the notochord, a proliferation of the cells of the external 

 epiblast takes place, forming a small, inwardly projecting knob. 



vv 



FIG.' 119. Transverse section across the head of a Chick Embryo at the forty- 

 third hour of incubation. The section is taken immediately behind the 

 auditory pits and the heart, and passes through the rudiments of the glosso- 

 pharyngeal nerves, x 1 00. 



A, aorta. BH, cavity of hind-brain. CH, notochord. E. epiblast. H, hypoblast. 

 ME, somatopleuric layer of mesoblast. MH. Bpianobnopleorlc layer of niesoblast. 

 NB, neural crest ; the part shown in the figure gives rise later on to the ganglia of the 

 glosso-pharyngeal nerves. W, vitelline vein. 



The nerve soon comes in contact with this knob, and fuses with 

 it, close to its distal end. Cells are budded off from the knob 

 into the nerve, which thus becomes reinforced from the epiblast. 

 The exact fate of these cells is uncertain, but it is probable that 

 they take part in the formation of the ganglioiiic thickening on 

 the nerve. 



The inner or proximal end of the nerve thins rapidly, and 

 loses its connection with the dorsal surface of the brain, a con- 

 nection which from the first has been one rather of close contact 

 than of actual continuity. A little way beyond this point, how- 

 ever, the nerve acquires its permanent attachment to the brain, 



