CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEA. 431 
c. The Mesoblast. 
We have already fully considered the origin of the meso- 
blast cells and their characteristic properties. Only little 
remains to be added. 
Once freely moving about in the blastocel they very soon 
accumulate against the inner surface of the plates of secondary 
epiblast, in the commencement retaining their more massive 
shape (I. c. (30), pl. ii, figs, 23 and 27; pl. iv, fig. 61), bat 
gradually flattening out and diminishing in size. Thus, at the 
same time the difference between epiblast and mesoblast cells 
becomes less and less marked. Dorsally the mesoblast cells 
have a very distinct tendency to unite into a separate layer of 
flattened cells (1. c. (30), pl. 11, figs. 29 and 30; pl. iii, fig. 50; 
pl. iv, fig. 68), which, of course, gives a great distinctness to 
the three different embryonic layers (fig. 6). The cells which 
are laterally added to this mesoblast layer, and thus constantly 
tend to extend its surface, show very instructive transitional 
forms between the more massive free mesoblast cells and the 
flattened ones composing the layer. There is a special accu- 
mulation of mesoblast cells in the prostomium, where they very 
soon fill up the space between the coalescing cephalic plates of 
secondary mesoblast, and surround the incipient proboscis, 
the inward growth of which has been described above (figs. 9, 
11, and 12). 
This accumulation of mesoblast cells is, at the same time, 
the first step towards the differentiation of tissues other than 
epithelial in the larva. We have already in the preceding 
pages hinted at the fact that the nervous system in Lineus is 
of mesoblastic origin; this must now be demonstrated. We 
very soon meet with a further differentiation amongst those 
mesoblast cells, which we find applied against the coalescing 
plates of secondary epiblast, i. e. against the larval integument, 
and which form a massive group in the prostomium, a compara- 
tively thin cell-sheet in the rest of the body. The process of 
differentiation of these embryonic cells into (1) muscle-cells 
