632 "Journal of Comparative Neurology and Psychology. 



The theory which I wish to advance is explained in the diagrams, 

 fig. 9. A represents a typical hemisection of the spinal cord; the 

 two sensory components of the nerve are represented by fine lines, 

 the visceral motor component by the dotted line, and the somatic 

 motor components by heavy lines. It w^ill be seen that the only 

 difference bet\wen this theory and that of Johnston is in the sub- 

 division of the somatic motor component into two parts which I 

 shall call the ventro-mesialand the ventro-lateral components, from 

 the position of their respective nuclei of origin in the ventral horn 

 of the cord; the reason for this subdivision will appear presently. 



Let us turn for a moment to the origin of the muscles which 

 these components supply, taking for a theorem that muscles with 

 the same embryological derivation will be innervated by the same 

 components. Corning (1899. i) in his work on the origin of the mus- 

 cles in amphibia, gives several figures of the developing myotomes, 

 and he and many others agree that the muscles of the back, except 

 those overlying ones connected with the shoulder and pelvic girdles, 

 develop from the cells in the dorso-latcral portion of the myotome, 

 while the muscles of the sides and ventral part of the trunk develop 

 from the ventral cells of the myotome. I wish to emphasize the 

 lateral position of the dorso-lateral cellsof the myotome. Van Bis- 

 SELiCK (1905. i) in a paper on the innervation of the trunk myotome 

 in Acanthias andMustelus, finds three distinct divisions of the myo- 

 tome, a posterior, a lateral, and a ventral division, each supplied by 

 a distinct branch or group of branches of the trunk nerve. The 

 lateral division of the myotome would be in a position to supply 

 cells to form the musculature of the limb. Corning and many 

 others regard the muscles of the limbs as developing from muscle 

 buds springing from the ventral part of the myotome, and there- 

 fore comparable with the ventral body muscles; Bardeen and 

 Lewis (1901.1), on the other hand, state that in human embryos 

 the muscles of the limbs arise from mesenchymal cells /« situ, 

 which have no connection with the myotomes. Many papers, 

 pro and con have been written, notably by Byrnes (1898. i). 

 Field (1894.1), Valenti (1902. i) and Kaestner (1893. i), and 

 it seems best to me to agree w4th Kaestner in considering that 

 the hmb muscles of mammals arise from myotomic buds (as do 

 certainly those of amphibia and other lower forms), but at such 

 an early stage that the separate cells of the myotome are not 

 distinguishable from the mesenchymal cells through which they 

 migrate. 



