MORPHOLOGY OF EYE MUSCLE NERVES 93 



preparations were not suited to demonstrate the histogenesis of 

 the neurofibrillae. 



Filatoff ('07, p. 343) finds that the anlage of the abducens in 

 reptile embryos appears primarily as a strand of mesenchyma- 

 tous cells extending from the posterior rectus muscle towards 

 the base of the medulla and as yet unconnected with the brain. 

 The cellular strand is not fibrillar. How connection with the 

 brain is effected, Filatoff is not able to state. 



All other students of the histogenesis of the abducens agree 

 that the cells of the abducens anlage have no genetic relation 

 to it but are secondary and accessory. The fibers of the anlage 

 arise as processes of medullary neuroblasts situated in the so- 

 matic motor column of the hindbrain, posterior to the otic 

 vesicle. All have noted the greatly elongated nidulus, which, 

 according to the writer ('98) extends through the first and 

 second post-otic neuromeres (neuromeres VII and VIII). Belo- 

 golowy ('10 a, p. 270) makes the interesting discovery that in 

 a 2.2 mm. embryo chick ''the roots of the abducens extend 

 through as manj* as five neuromeres." Later they arrange them- 

 selves into three groups, which however show no regular rela- 

 tion to the neuromeres. By a comparison of the relations of 

 the abducens anlage to the neuromeres, Belogolowy concludes 

 that the most anterior roots of the abducens of the chick take 

 their origin from a neuromere next anterior to the one from which 

 they originate in the dogfish (Squalus). Posteriorly the roots 

 of the embryonic abducens in the chick are connected with those 

 of the hypoglossus. . The posterior roots of the abducens show 

 a marked tendency to grow backwards as if to join those of the 

 hypoglossus which lie immediately behind them. Still later some 

 of the roots degenerate. Another important discovery made by 

 Belogolowy is that of the existence of a transient somatic motor 

 nerve, ventral to the ramus maxillaris trigemini and uniting 

 peripherally with the oculomotor with which the abducens also 

 unites. The inference of the medullary derivation of the neu- 

 raxons of the abducens is based on the evidence of the connec- 

 tion between deeply staining cells of the medulla and the fibers 

 of the nerve anlage, from the time of the first appearance of the 



