60 H. V. NEAL 



HISTOGENESIS OF THE EYE MUSCLE NERVES 



Does the histogenesis of the eye-muscle nerves resemble that 

 of spinal somatic motor nerves, and are they therefore morpho- 

 logically comparable? Upon the answer to this question depend 

 in very large degree our views of the morphology of the verte- 

 brate head. 



1. General description of the histogenesis of the eye muscle nerves 



The three eye-muscle nerves make their appearance at differ- 

 ent stages in the development of the embryo; the oculomotor in 

 a 9.5 mm. embryo; the abducens in a 10 mm. embryo; and the 

 trochlearis in a 19 mm. embryo. They arise like spinal somatic 

 motor nerves as plasmatic protrusions from the wall of the brain. 

 Their terminations are amoeboid and remain so until connection 

 with the muscle is established. The primary anlagen possess a 

 peripheral layer of clear protoplasm of somewhat granular struc- 

 ture, within which may be detected an axial deeply-stained fiber 

 (fig. 32). The thickness of the peripheral protoplasmic sheath 

 varies in inverse ratio with the length of the fiber. As the nerve 

 anlage elongates the granular sheath becomes exceedingly thin. 

 The trochlear, from the time of its first appearance, in contrast 

 with the abducens -and oculomotor, appears fibrillar rather than 

 protoplasmic, owing to the delicacy of the plasma film surround- 

 ing the axial fibers (figs. 49 and 50). This difference may be 

 attributed to the extended course of the fibers of the trochlear 

 within the brain wall. 



Evidence of a primary plasmatic path extending from the 

 amoeboid terminations of the eye muscle nerve anlagen to the 

 myotomes with which they are later connected is lacking. The 

 demonstration of their presence or absence is rendered difficult, 

 however, as a result of the presence of a loose mesenchyma in 

 the spaces traversed by the elongating nerve anlagen. The an- 

 lagen assume close relations with mesenchyma cells and their 

 processes, but so far as may be observed these connections are 

 secondary and are independent of the process of elongation of 

 the nerve anlagen. In all cases where it is possible to identify 



