CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTEA. 419 
Lineus does not arise out of four but out of five discs. 
The formation of the fifth disc commences but very little later 
than that of the four just mentioned; it, however, arises in a 
different manner. In the aboral region of the epiblast a very 
distinct process of delamination of the epiblast cells sets in 
(fig. 3), and the inner layer of cells remaining in connection 
with the outer (figs. 4, 5), a double layer thus originates, 
which finally separates, not, however, simultaneously along its 
whole surface, but first only in the middle (fig. 6), the dorsal 
layer of secondary epiblast being thus connected with the 
primary in the same manner as obtained in the paired lateral 
discs of secondary epiblast. All the five discs are one cell- 
layer thick ; they increase in size by continued division of the 
constituent cells, and perhaps, also, by further participation of 
the primary epiblast along the line which marks the circum- 
ference of the discs ; they finally meet along their edges; they 
then unite (fig. 9), and form the continuous coat of secondary 
integument, outside of which the primary epiblast is now only 
temporarily retained, and is very soon cast off. 
Whereas we have seen that the fifth plate was decidedly 
dorsal in position, we might term the two lateral pairs the 
cephalic plates and the ventral plates. The former are situated 
before, the latter behind the blastopore (Pl. XXII, fig. 10), and 
a section through the blastopore (as in fig. 4) was often obtained, 
in which neither the cephalic nor the ventral pair of discs had 
been cut, and in which only the dorsal disc was visible. Hori- 
zontal (fig. 10) and longitudinal (fig. 7) sections reveal the 
presence of yet another centre of delamination in the primary 
epiblast, which is situated at the anterior pole. This delamina- 
tion, which has been traced by me in all its successive phases 
(and more elaborately figured (30), pl. iv, figs. 55—59), is the 
first origin of the proboscis. It soon separates from the 
primary epiblast in its middle portion, which elongates and 
grows rapidly backwards (fig. 9), whereas the outer circum- 
ference fuses with the cephalic discs when these reach forward 
and meet along the median line (30) (pl. iii, figs. 43 and 44). 
From this moment it is no longer possible to perceive that the 
