305 



of greater extent than has generally been recognized. Secondly, as 

 already stated, these migrant cells are to be regarded as the indifferent 

 cells of ScHAPER. This conception immediately opens the way for 

 ä priori speculation as to their fate. Inside the neural tube some in- 

 different cells develop into supporting ^. -__^ ^ 

 cells, some into nerve- cells. Are there / .§ 

 similar transformations outside the neural 

 tube? 



The studies of GuRWiTsbn ('00), 

 Bardeen ('03), and others leave no doubt 

 that the elongated cells found among the 

 fibers of developing nerves become the 

 cells of the sheaths of Schwann. Some 

 of these elongated cells probably come 

 from spinal ganglia, as Harrison ('06) 

 has proved experimentally in the case of 

 frog larvae; some may, possibly, be 

 derived from the surrounding mesenchyme; 

 but others we feel sure, are escaped 

 medullary elements. Such cells subserve 

 a supporting function, which may be 

 compared in a general way to that of the 

 neuroglia - cells of the central nervous 

 system. 



To decide whether or not any of 

 the migrant indifferent cells become the nerve-cells of peripheral, that 

 is, sympathetic ganglia is far more difficult. Not until a staining 

 method is devised which will unmistakably differentiate the indifferent 

 cells from mesenchyme cells can one hope to settle this question by 

 the study of sections. Harrison ('01), who saw medullary cells 

 migrating into the ventral nerve- roots of salmon embryos, has already 

 suggested the possibility that these cells may pass along the visceral 

 branches of spinal nerves into sympathetic gangha, and there give 

 rise to motor neurones. This would place in direct genetic relation 

 motor neurones of the sympathetic system and motor neurones of the 

 cerebro-spinal system. In connection with this question of the trans- 

 formation of migrant indifferent cells into nervous elements it is 

 interesting to note that Schäfer ('81) and Koelliker ('94) have 

 described ganglion cells in the ventral nerve-roots of adult cats. In 

 a former paper (Carpenter '06) it has been shown that some of the 



Diagram of Cells of Em- 

 bryonic Mammalian Spinal Cord. 

 (Modified from ScHAPER, '97.) 

 Elongated dotted cells, germinal 

 cells ; circular cells with crosses, 

 germinal cells in mitosis ; circular 

 and elliptical cells without mark- 

 ings, indifferent cells ; circular 

 cells with dots, neuroglia- cells; 

 block cells, nerve-cells. 



Aiiat. Anz. XXXI. Aufsätze. 



20 



