of the rudinent and those of the tube, as seen in Pig. 51, a. 



]"[any sections, however, such as the one shov/n in 

 Fig. 31, _b, v/hich represents a slightly older stage, leave 

 little room for doubt that nuclei do wander out into the ru- 

 diment. But, on the other hand, I think that this figure 

 shows equally well that mesodermal cells are added to the 

 mass from the outside. 



I have, therefore, come to the conclusion that the 

 ganglion has a double origin, and that both the wall of the 

 tube and free mesodermal cells co-operate in forming it. 

 It is to be remembered, however-, that ultimately it is entire 

 ly a mesodermal structure, as we have seen hov/ the dorsal 

 tube is constructed out of cells of the blood. 



The ganglionic- rudiment is at first a very irreg- 

 ular heap of cells, and is clearly associated at the periphe- 

 ry with surrounding mesoderm.al cells. The cell-boundaries 

 are completely lost very early, and the mass rapidly increases 

 in size by multiplication of nuclei within, by further ac- 

 quisition of cells from without, and by continued migration 

 of nuclei from the wall of the tube. Pig. 31, c. The nuclei 

 now arrange themselves in a couple of layers around a central 

 core, in which fine fibrils are laid dov/n, and the ganglion 

 becomes completely marked off from tne v/all of the tube; the 

 definitive cti'ucture is nov/ attained. 



'"ig. 31, d and _e, illustrates the latter course of 

 development . 



?;Iy observations, therefore, have led me to believe 



