240 The Development of the Neuroglia 



up by the further extension of the periphery, and by the development, 

 ingrowth and further elaboration of the nerve-elements and blood-vessels, 

 nuclei invade the mantle layer both from the nucleated layer (ecto- 

 dermal) as well as from the outside (mesodermal). The latter nuclei 

 may either accompany the walls of the ever-extending blood-vessels or as 

 independent ingrowths of the developing pia mater. This double inva- 

 sion goes on till, in pigs of 8 to 10 centimeters, the mantle layer appears 

 almost as thickly nucleated as any other part of the section. The nuclei 

 are later dispersed in the final rapid thickening of the mantle layer in 

 consequence of the medullation of the axones coursing in it. 



Only at first is the origin of all the nuclei of the section strictly ecto- 

 dermal, though at all times perhaps a good majority of them are of this 

 origin. In all the earlier stages the mitoses giving rise to the nuclei 

 oJ' this origin all occur in the inner zone of the section, or the most ven- 

 tricular portion of the wall of the neural tube and just within the mem- 

 hrana limitans interna. All the divisions are by the indirect method, and 

 in almost every case the long axis of the polar spindle lies parallel to the 

 membrane or transverse to the general radial structure of the tube. In 

 the process the nuclei become much swollen and bubble-like and, in the 

 middle phases, the chromatin filaments are surrounded by an area of 

 less stainable substance — a clear court. In the early stages, when the 

 cell-membranes are more or less evident (Figs. 1 and 2), the phases of 

 division involving the membrane may be observed. In the later stages 

 there are no real cell-membranes. What at first appear as such, when 

 followed through the phases, seem nothing more than a zone of the 

 more stainable protoplasm packed away from the center of activity dur- 

 ing the swelling of the nucleus (Figs. 3 and -i). This resemblance to 

 a cell membrane is exactly similar to that which appears about dividing 

 nuclei in the connective-tissue syncytium outside the central nervous 

 system, where individual cell-membranes are impossible (see Fig. 5). 



After division, the nuclear membrane reforms and at least one pro- 

 duct of the division begins to migrate toward the periphery. In the 

 process of migration the nucleus becomes slightly elongated and, as it 

 leaves the limiting membrane, the clear court it occupies usually becomes 

 pointed and then gradually disappears and, accompanying its collapse, 

 the surrounding protoplasm is radially drawn out and, usually by means 

 of it, the nucleus, for a time at least, remains in direct connection with 

 the internal limiting membrane. A great majority of these nuclei, in 

 the further migration, become so dispersed as not to appear interposed 

 in the radial filaments but are merely enmeshed in the less accumulated 

 portion of the syncytium between the filaments. 



