234 The Developmoiit of the Neuroglia 



Figs. 1 to 4 are given to show tlic beginning and primitive i'orni of 

 the syncytium. Being unable to obtain a pig embryo in the stage 

 either before or at the closing of the neural tube. Fig. 1 is a copy of a 

 drawing from His, 89, and represents the medullary plate of a rabbit 

 embryo just before the closing of the tube. All the other figures are 

 from my series of pig embryos. 



Fig. 1 shows the M-all of the nervous system when it consists of but 

 one layer of cells. These are distinctly outlined with their boundaries 

 intact. The nuclei are so ])laccd that on the dorsal side, or what will 

 be the ventricle after the closure, there appears a zone of cytoplasm (a) 

 thicker ilian at the ])ei'iphery (m). All the nuclei in mitosis (g), or 

 germinal cells of His, are situated in the wider or ventricular zone. 

 Neither of the limiting membranes are as yet evident. 



Fig. 2 represents a stage (5 millimeter pig) after the tube has closed 

 and after considerable cell-division has occurred in the walls of the tube. 

 The nuclei are irregnlarly distributed in at least three rows. All nuclei 

 showing karyokinesis (//) are sitnaied in the venti'icidar zone {a). 

 Throughout tlie section cell-membranes are rapidly disappearing, except 

 those of the cells immediately bordering the ventricle. Tho membranes 

 of the long axes of the cells persist longer tlian at the ends. Tlie oblitera- 

 tion of the boundaries of tlie ends of the cells results in radially ari-anged, 

 nucleated columns of protoplasm (r), extending from the ventricle to 

 the periphery. Throughout the protoplasm of the section a general 

 spongioplasmic network is easily seen. It is not interrupted along 

 lines where oiu^ would judge cell-membranes have recently existed, and 

 its filanienls are somewhat coarser than one would expect from the 

 study of other cells. 



While the general epithelial character is still maintained at this stage, 

 it is noticeable even here that the nuclei are becoming so arranged as 

 to give the a])pearance of three zones or layers in the section : — an inner 

 zone (a) practically free from nuclei other than those in the phases of 

 mitosis; a middle, nucleated layer, and an outer layer (m), into which 

 nuclei do not extend. It seems that the absence of nuclei in both 

 the inner and outer layers is due to the nuclei of the cells forming the 

 layers being sitmiled in the ends of tlie t'clls fartliest away from the 

 inner and outer surfaces of the specimen. This three-layered appear- 

 ance is maintained in the later forms by the migration of the nuclei 

 from the inner layer, where they originate, into the middle nucleated 

 layer, where they add to its thickness. One of the " germinal cells " 

 (<7, Fig. 2) is probably beginning to migrate. 



The internal limiting membrane appears before the external. It is 



