MORPHOLOGY OF EYE MUSCLE NERVES 49 



position as the result of protoplasmic migration from the neural 

 tube, there is equally good reason for the conclusion that its 

 neuraxon has been formed by the same method. 



7. What is the source of the cells of somatic motor cells? 



The cellular structure of embryonic somatic motor nerves has 

 been emphasized by many embrj^ologists since Schwann ('39) 

 advanced the cell-chain hypothesis of neurogenesis and since its 

 subsequent revival by Balfour ('77). The question of the source 

 of these cells has never been fully determined. Are they medul- 

 lar}' or mesenchymatous in origin? 



Balfour ('75) was the first to infer the migration of medullary 

 cells into somatic motor nerve anlagen and this conclusion has 

 been confii-med by many investigators including Dohrn ('88), 

 Beard ('88j, Herrick ('93), von Kupffer ('94, '95), Hoffmann 

 ('97), Harrison ('01), Bardeen ('03), Xeal ('03), Schultze ('04- 

 '07), Held ('06, '09), Carpenter ('06), Carpenter and Main ('07) 

 and Kuntz ('10-'12). His ('89) and von Kolliker ('92), on the 

 other hand, have denied the derivation of these cells from the 

 neural tube. The former concluded that the cells which appear 

 to be in the process of migration from the tube, are later enclosed 

 b}' the 'Randschleier' which prevents their escape. If it were 

 otherwise, he thinks, the process would lead to the formation of 

 motor ganglia. To His it . appeared quite inconceivable that 

 connecti^•e tissue elements could come from the nervous system. 

 The mesenchjmiatous origin of the cells of somatic motor anlagen 

 has also been assumed by Vignal ('83), von Lenhossek ('97), 

 Kolster ('99), Gurwitsch ('00) and Bardeen ('03). Von Kolliker 

 ('05) came finally to accept the probabihty of the medullary 

 origin of the neurilemma. According to ^Marcus ('09), the cells 

 of somatic motor nerve anlagen come from the neural crest. 



The fact that sclerotome cells are in close proximity to the 

 somatic motor anlagen has led some students to infer that the 

 cells present in later stages within the nerve have had a mesen- 

 chymatous derivation. In fact, the mesodermal origin of all 

 connective tissue cells has seemed so well attested that many 



JOURXAL OF MORPHOLOGY, VOL. 25. XO. 1 



