Irving Hardesty 237 



arrangement of the nuclei is disturbed, the nuclei being less oval and 

 some of them may even lie transverse to the general disposition. This 

 variation is coincident with a change in the behavior of the axial 

 masses as they are continued into the mantle layer — a change which 

 becomes more pronounced in the later stages. The axial masses, on 

 reaching the border line between the nuclear layer and the non- 

 nucleated mantle, apparently bifurcate, and the bifurcations give rise 

 to a more tangled complex of the resulting threads. In the later stages 

 it will be seen that some of the threads may even course at right angles 

 to the general arrangement (Figs. 7 to 11). The tangle thus produced 

 in this locality apparently becomes resistant enough to prevent, for a 

 time, the nuclei from migrating into the mantle layer. 



While the bifurcations of the radially disposed threads may be looked 

 upon as a means of compensating the naturally greater dispersion of 

 the substance at the periphery of the tube, yet they do not arise by a split- 

 ting of the threads as one might suppose from their appearance in the 

 later stages (see Fig. 7). The bifurcations arise in practically the same 

 way as the threads themselves. The method of their origin may be 

 determined by comparing the structure of the mantle layer (m) in Figs. 

 2, 3, 4 and 7. The fine-threaded, small-meshed reticulum of the earlier 

 stages grows into a coarser, larger-meshed form. Then there appear 

 numerous, still larger meshes in the reticulum, and the smaller-meshed 

 arrangement bounding these larger openings gives the appearance of 

 irregularly arranged reticulated trabeculse (Fig. 3). As the wall of the 

 neural tube further grows in thickness, the mantle layer also thickens, 

 and in the process undergoes a change in structure. The reticulated 

 trabeculse increase in substance, but, at the same time, being attached 

 to the ever-extending periphery, they are drawn out or condensed till 

 the meshes within them are obliterated and they become apparently 

 solid trabeculse with numerous thinner lateral branches extending, as 

 before, into the large meshes (Fig. 4). Let this process of radial draw- 

 ing out of the syncytium and condensation of the smaller meshes con- 

 tinue, there begins to be suggested a radial arrangement in the mantle 

 layer itself (see figures of the later stages). Thus the radially 

 arranged axial masses of the inner portion of the tube being continuous 

 with the protoplasm of the mantle layer, their apparent bifurcations 

 result in the course of the change in the form of the mantle layer. 



Just as in the development of the nervous elements, the ventral por- 

 tion of the neural tube precedes the dorsal portion (see Fig. 5), so, at 

 first, the syncytial framework grows more rapidly in the ventral por- 

 tion. It is on the ventral periphery that the mantle layer first thickens 

 18 



