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ALBERT KUNTZ 



occupied by the fibers of the dorsal nerve-roots, and cells are 

 rarely found among them (fig. 4, d.n.r.). Migration of medullary 

 cells into the dorsal nerve-roots evidently ceases before the 9 mm. 

 stage is reached. It is probably only a transient process which 

 takes part in the development of the cerebro-spinal ganglia. 



In transverse sections of embryos from 6 to 11 mm. in length, 

 breaches occur frequently in the external limiting membrane 

 in the region of the ventral nerve-roots. Medullary cells migrate 

 in considerable numbers, through these breaches, into the ventral 

 nerve-roots (fig. 5, c.m.v.r.) . This fact has recently been observed 

 by Carpenter and Main ('07). I have been able to substantiate 



sprri 



LCC.T 



i 



• 9 



f 



Fig. 6. Section showing the path of the communicating ramus in an embryo 

 7 mm. in length, X 190. 



their observation that cells may be observed "just inside the 

 external limiting membrane, in an intermediate position half 

 in and half out of the neural tube, and in the base of the ventral 

 nerve-root just outside the external limiting membrane." 



The orientation of the cells in the neural tube is such, during 

 the period of migration, that the cells which migrate into the dor- 

 sal and the ventral nerve-roots seem to have their origin in more 

 or less distinct regions. In the dorsal region most of the migrat- 

 ing cells move quite directly outward from the dorsal zone, while 

 others tend dorso-laterally in slight curves from regions ventral 

 to the dorsal nerve-roots. In the ventral region some of the 



