DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 909 
ectodermic ridge would be lost. In the head, cells that mi- 
grated from the neural crest or thickened ectoderm were easily 
distinguished from surrounding tissues by difference in the 
size of the yolk granules they contain; but in the trunk, at 
this early stage, cells of the skin and spinal cord are not more 
free from yolk than those of the protovertebra, nephrotome, or 
lateral plate. 
Fig. 11 gives the comparative distribution of yolk granules 
through the tissues of the trunk. The first cell differentiation 
occasioned by reduction of the yolk granules occurs in the 
ventral part of the spinal cord, and gradually extends through 
the cord dorsalwards. Some of the cells from the neural crest 
which lie in the loose mesenchyme at the side of the spinal 
cord also soon become clearer than the more ventral mesen- 
chyme cells that have possibly come from the lateral plates, 
but there are always intermediate cells in regard to whose 
origin the yolk granules furnish no clue. Therefore, should 
cells from the median ridge join those migrating at the same 
time from the lateral plates, I know of no means by which the 
ectodermic cells may be identified. 
On reviewing my sections, however, I find that in the region 
where the intersegmental ridge joins the median longitudinal 
ridge, and where as yet no mesenchyme cells from below have 
wandered, cells may be observed to migrate from the ectoderm. 
In Pl. 36, fig. 8, I have represented such a cell. The section 
is from the same series as fig. 9, but its plane is two segments 
anterior to that of fig. 9. The migration of cells from the 
neural crest above the second permanent protovertebra (fig. 8) 
has not extended as far as the fourth protovertebra represented 
in fig. 9. The intersegmental ridge is a dorsal extension from 
the median longitudinal ridge, and being deepest where it 
meets the median ridge, gradually fades away as it rises be- 
tween the protovertebre. As development proceeds, however, 
the longitudinal ridges of the trunk become less distinct, while 
the intersegmental ridges gain in prominence, thus repeating 
in the trunk the sequence of the ectodermic ridges occurring 
in the head. The cell migration actually observed in favor- 
VoL. 38, PART 4.—NEW SER. MM 
