Arthur B. Lamb 



199 



as in the anterior portion, cells are assuming an elongate form and a 

 longitudinal direction. The posterior end is indistinct in outline, and 

 cells are evidently dropping off into the mesenchyme. By this degener- 

 ation at its posterior end, by growth of the muscle as a whole, and 

 especially by the outpushing at its anterior end (Fig. 9, E), the whole 

 somite moves forward, so that while originally located some distance 

 away, it comes to lie in close proximity to the eyeball (Figs. 4, 5, 7, 8, 

 E. Bee. Fig. 9, a). 



OP ve 



& »8P*'S 









Fig. 9. — Part of transverse section of Acantbias embryo, 19-20 mm. total lengtb, 

 showing tbe proliferation of the external rectus muscle (E) from the byoid (III) 

 somite. 



This somite is innervated by the abducens. This nerve is, as men- 

 tioned above, generally considered as homologous to the ventral motor 

 root of spinal nerves. It arises (Neal, g8, p. 230) at a ten mm. stage 

 from the tloor of the hind brain at a point opposite the ear vesicle as 

 an outgrowth from neuroblast cells in the ventral wall of eneephalomere 

 YII. The nerve gradually extends forward until it reaches its somite. 



