236 The Development of the Neuroglia 



sarily thin sections, they do not appear thronghoiit. However, many 

 of the nuclei are not interposed in these threads at all, hut are merely 

 enmeshed in the finer reticulum. This is especially true for the more 

 scattered nuclei situated in the outer portion of the nucleated layer. 



The outer, non-nucleated layer (m) is beginning to assume the form 

 which later characterizes it as the mantle layer or Randschleier of His. 

 From the fine reticulum in Fig. 2, its protoplasm is now arranged into 

 reticulated partitions bounding larger meshes, or in other words, it 

 has become a sort of reticulated reticulum, a net seemingly with irregu- 

 lar areas of broken meshes. At the periphery the flne-meshed reticulum 

 is more intact and a portion of it is condensed to form the now distinct 

 membrana limitans externa (nile). 



The connective-tissue syncytium immediately surrounding the neural 

 tube (p) is at this stage becoming arranged with its meshes parallel 

 to the external limiting membrane. Its anastomosing fusiform and 

 stellate masses (cells) are becoming stretched upon the surface of the 

 tube, due perhaps entirely to the growth of the latter, and thus there 

 results the first appearance of the membrana meningea. At this stage 

 no blood-vessels have grown into the spinal cord, and of course there are 

 no nerve fibers. 



Fig. 4 represents a ventrolateral segment from a transverse section 

 of the neural tube of a pig embryo measuring 10 millimeters (flexed). 

 The conditions found in this stage are easily seen to be transition 

 forms of those shown in Fig. 3. Both of the limiting membranes have 

 gained in thickness and the general radial arrangement of the syncy- 

 tium between them is more marked than at 7 millimeters. 



On close examination of the inner zone (a) what, under low power 

 and especially in thicker sections, appears to be a layer of narrow cells, 

 is really but the now evident ventricular ends of the axial threads 

 formed by a spinning out of the protoplasm in the inner limbs of cells 

 which Avere more or less distinctly bounded by membranes in Fig. 3, 

 but which membranes have now entirely disappeared. The drawing 

 out has resulted in the collapse of the original fine-meshed reticulum 

 here, and now all that remains of it is represented by the fine lateral 

 branches connecting the axial masses. As before, all divisions of nuclei 

 occur in this zone and from the continued migrations, the middle, 

 nucleated layer has gained further in thickness. 



The further drawing out of the s}Ticytial protoplasm gives rise to a 

 more marked and coarser radial arrangement in the nucleated layer 

 and to an interesting modification of the non-nucleated mantle layer 

 (m). At the outer margin of the nucleated layer (&) the general 



