Irving Hardesty 335 



evident in pigs of 5 millimeters (inU, Fig. 2). A study of their for- 

 mation lends the impression that both of these membranes result from 

 first a fusion and then a narrow condensation of the protoplasm imme- 

 diately bordering the surfaces of the tube. The mesenchymal tissue 

 surrounding the tube is already in the state of a eouipletoly formed 

 syncytium, though the formation of the embryonic meninges from it is 

 as yet scarcely begun. 



Fig. 3 represents a lateral segment of a transverse section of the 

 spinal cord of a pig of 7 millimeters (practically unflexed). The 

 arrangement into three layers is more evident, though the middle, 

 nucleated layer is much thicker than in Fig. 2. The formation of the 

 syncytium is now almost complete. No cell boundaries arc evident, 

 except in the inner layer (a), which is nothing more than the remain- 

 ing inner limbs of the cells bounding the ventricle. As in Fig. 2, this 

 layer contains no nuclei other than those in phases of karyokinesis. 

 The middle, nucleated layer is thickened, and its nuclei show a radial 

 elongation. The syncytium once formed, there must be less resistance 

 to the radial migration of the nuclei from their layer of origin. In 

 fact, the very movements of the nuclei perhaps aid in producing the 

 syncytium. The movements evidently play a role in its later arrange- 

 ment. 



The spongioplasmic reticulum is much coarser than in Fig. 2, show- 

 ing both coarser and larger meshes. The cell boundaries having dis- 

 appeared from the unexposed ends of the cells of the ventricular layer, 

 the reticulum of these cells is continuous with that of the general 

 syncytium of the section. The radially arranged columns apparent in 

 Fig. 2 (r), if represented in Fig. 3 at all, have undergone great attenu- 

 ation and appear drawn out into axial threads of more densely accumu- 

 lated protoplasm (r. Fig. 3), in which several nuclei may be interposed. 

 These axial threads seem to result from a spinning out of the reticu- 

 lated protoplasm, due to the direction of growth and the movements 

 of the nuclei through it. Thus radially arranged, the threads remain 

 intimately connected with each other by means of numerous finer fila- 

 ments, and the whole go to form a general reticulated syncytium with radi- 

 ally elongated meshes. At one or either end of the nuclei there is natur- 

 ally more of the protoplasm than in the general diameter of the threads. 

 This, if the extent of the threads is not realized, may give the appear- 

 ance of conical or fusiform cells scattered through the section. That 

 the axial threads seldom appear continuous through the nuclear layer 

 is no doubt largely due to their intermingling among themselves instead 

 of maintaining a straight radial course and, consequently, in the neces- 



