404 WILSON. [Vo. III. 
In early stages all the cell-rows are but one cell in width at 
their hinder ends. Passing forwards they soon become several 
cells wide and deep, and the two nephric rows on each side fuse 
completely. Only the neural and the mesoblastic rows can be 
traced around the mouth; the others fade away at the sides of 
the stomodeum. As the embryo enters the later stages of 
development the teloblasts break up into smaller cells, so that 
each cell-row, now several cells in width throughout its whole 
length, terminates behind in a group of undifferentiated cells 
(Figs. 63,97). The neuroblasts, nephroblasts and lateral telo- 
blasts first disappear ; the primary mesoblasts persist to a much 
later period, and only disappear near the time of hatching, when 
the proctodzeum is formed. 
Concrescence of the bands begins in front and proceeds 
thence regularly backwards, the union of the bands taking place 
first in the median ventral line, afterwards in the median dorsal 
line, as the mesoblastic bands extend around the archenteron. 
It begins at a very early period, as already described, with the 
anterior fusion of the ectoblastic part of the bands to form the 
stomodzeum. The mesoblastic bands move together at a much 
later period. As the bands come together the neural rows of 
the two sides come into contact in the middle line, fuse more 
or less completely together, and thus give rise to the ventral 
nerve-chain. Posteriorly the bands diverge up to a very late 
stage of development, having the disposition shown in Fig. 63. 
Owing to this divergence and the subsequent junction of the 
bands at the extreme posterior end, a somewhat elongated space 
is enclosed by the hinder part of the bands, in which the ecto- 
blast and entoblast are directly in contact. 
From the foregoing account it will be seen that the cephalic 
lobe (prostomium, or head) arises by concrescence of the an- 
terior extremities of the same germ-bands that form the trunk. 
A more detailed account of the formation of the head is given 
at page 407. 
VI. GENERAL HISTORY OF THE MESOBLAST. 
1. General Differentiation. — The forward growth and union of 
the mesoblastic bands to form the cephalic mesoblast has already 
been described. Upon the completion of this process the meso- 
