86 WHEELER. [Vol. VI 1 1. 



the neuroblasts and continuous laterally with the general ecto- 

 derm. In the median line they extend to the deepest portion 

 of that germ-layer in the form of a few compressed cells (Fig. 

 26, db). These compressed cells form the walls and bottom of 

 the neural furrow. The proliferation of the neuroblasts has 

 caused the lateral cords to bulge out enormously (Fig. 27), so 

 that the dermatoblastic layer becomes stretched and attenuated. 

 Such divisions as occur in the cells of this layer seem to be 

 confined to the outer surface and do not extend into the furrow. 

 The spindle axes lie parallel to the surface, as shown at nb. 

 The bulging of the lateral cords naturally brings about a deep- 

 ening of the neural furrow, since the cells at its bottom have a 

 fixed attachment. At this point in Fig. 27 there is seen a 

 triangular cell-mass, capped by a single large element (innb.), a 

 true neuroblast which resembles in nearly all respects the 

 neuroblasts of the lateral cords. Its more pyramidal outline is 

 obviously the result of its position between the converging 

 walls of the furrow. To the same mechanical cause is due the 

 shape of the cell-mass, which consists of the heaped up 

 daughter-cells of the neuroblast. Inasmuch as the proliferating 

 cell occurs in the median line, and together with its offspring 

 and the dermatoblastic cells of the median furrow, is equivalent 

 to the "Mittelstrang" of authors, I shall call it the median- 

 cord neuroblast. Its exact origin I have not been able to 

 determine. To judge from the number of cells which it has 

 given off it must have begun to proliferate at about the same 

 time as the lateral-cord neuroblasts. There can be no doubt, 

 it seems to me, that it originated as a polygonal ectoderm cell 

 like the lateral cells seen in Figs. 25 and 26, but whether it 

 w:as originally median in position or arose unilaterally I am 

 unable to decide. The pale surface spots of embryos in Stage 

 B show that neuroblasts are arising at a time when the blasto- 

 pore occupies the position of the neural furrow and hence, if 

 the median cells are median in position from the first, they 

 must arise somewhat later than their sister neuroblasts. 



There is one important difference in the arrangement of the- 

 mother-cells of the lateral and median cord. Whereas the for- 

 mer, as has been stated, form continuous though irregular rows 



