MORPHOLOGY OF EYE MUSCLE NERVES 51 



connection with each other by fine plasmodesmatous threads. 

 Thus the study of serial sections of Squalus embryos in closely 

 related stages of histogenesis permits no doubt that the cells of 

 somatic motor nerve anlagen are in large part migrant medullar}^ 

 elements. 



From an estimate of the number of migrating medullary cells 

 and a comparison with the estimated number in adult nerves 

 the writer ('03) concluded that the neurilemma receives acces- 

 sions in later stages from the mesenchyma and that the mesen- 

 chymatous participation in the formation of the sheaths of the 

 adult nerve is greater than is the medullary contribution. The 

 reasons for this inference are more convincing if, as has been 

 suggested by Harrison ('01) and others, some of the migrant 

 medullary elements form the sympathetic anlagen. The writer 

 agrees with His, Jr. ('97) that at least a part of the cells of 

 somatic motor anlagen have a mesenchymatous derivation. 



8. What is the fate of the cells found in somatic motor yierve anlagen? 



Three views have been advanced as to the fate of the cells of 

 somatic motor nerve anlagen: 



1. They are 'nerve-forming cells' and they secrete the neuro- 

 fibrillae. 



2. They form the neurilemma cells. 



3. They become the ganglion cells of the sympathetic. 

 According to the first view the cells of somatic motor nerves 



anlagen form the nerves, either by fusion into cell-chains as 

 Schwann ('39) and Balfour ('78, '81) suggested and has since 

 been maintained by Marshall ('78), Van Wijhe ('82, '86, '89), 

 Beard ('85, '88, '92), Beraneck ('87), Goette ('88), Dohrn ('91), 

 Miss Piatt ('94, '96), Sedgwick ('94) with modifications, Hoff- 

 mann ('96), Kupffer ('90, '91, '94), Capobianco e Fragnito ('98), 

 Rafaelle ('00), Bethe ('00-'07) with modifications, Brachet ('05, 

 '07), Cohn ('05-'07) and Oscar Schultze ('04, '07); or as the 

 'nerve cells' which secrete the neurofibrillae and which attain 

 connections with ganglion cells and muscle fibers along plasmo- 

 desmatous paths without the participation of cell-chains. 



