34 H. V. NEAL 



erable. But this theoretical difficulty loses its force when ap- 

 plied to the connections of organs in such close proximity as 

 are neural tube and mytome in the trunk region of vertebrate 

 embryos. 



To summarize the evidence that connection between neural 

 tube and muscle segment is formed by protoplasmic movement 

 of medullary cells, we have first the fact that in early stages no 

 protoplasmic connections between tube and somite are found. 

 A plasmoid substance fills the space between tube and myotome. 

 Later, medullary cells in the ventro-lateral wall of the tube 

 develop a neuro-reticulum and manifest deeply staining proper- 

 ties. This change is followed by a protrusion of amoeboid 

 processes into the space between tube and somite. Later such 

 protrusions become wider and more extensive, several adjacent 

 cells adding to the size of the protoplasmic bridge thus formed 

 between tube and myotome. The movement of medullary 

 protoplasm is correlated with a migration of sclerotome cells 

 into the space between tube and somite but the latter elements 

 participate only temporarily, if at all, in the formation of the 

 ectoderm-mesodermic connection. 



In this manner are formed the anlagen of somatic motor 

 nerves, which at first appear non-fibrillar and protoplasmic 

 and entirely devoid of cells. These are the protoplasmic bridges 

 of Paton ('07). Later the neuroblastic processes extend 

 ventrad, along the median surface of the myotome and in close 

 contact with it and the sclerotome (figs. 10-11). Medullary 

 nuclei soon begin to wander into the anlage from the tube 

 and the movement becomes so extensive that the form of the 

 tube in cross section is greatly changed (fig. 12). All these 

 stages may readily be seen in cross-sections of a Squalus embryo 

 of Balfour's Stage I, by comparing sections beginning with 

 the cloacal region and passing forward toward the head. Con- 

 nections in the cloacal region at this stage are still unformed, 

 while the movement of medullary cells has already begun in the 

 more anterior metameres of the trunk. 



