CONTRIBUTIONS TO THE EMBRYOLOGY OF NEMERTBA. 425 
sacs, their shifting to a more lateral situation, and their 
definite, though secondary connection with the (secondary) 
epiblast takes place in early developmental stages, and that 
in those stages nothing is seen (in actual sections) of any 
connection with the cesophagus. 
The stages which I have described above as occurring later 
on during the development of the cesophagus of Lineus must 
somehow have misled even so accurate an observer as 1s 
Barrois. 
b. The Hypoblast before the Shedding of the 
Primary Larval Integuments. 
The general features of the process by which wandering 
mesoblast cells take their origin in the hypoblast have already 
been recorded above. 
We must now inquire more closely into the nature and 
significance of a phenomenon, the interpretation of which has 
given me very considerable difficulties. Whereas the commu- 
nication between the archenteron and the exterior, by means 
of a wide blastopore, is most evident in the earliest stages of 
development, I find without exception that in later stages (cf. 
fig. 7) the cavity in that portion of the intestine which com- 
mences to grow backwards is closed anteriorly, and that in 
front of this another portion of the embryonic intestine con- 
stantly remains in open communication with the exterior, but 
is never in communication with the posterior portion of the gut 
(figs. 8 and 9). We will provisionally call these two cavities 
the larval fore-gut and hind-gut. The anterior cavity or larval 
fore-gut opening outwards ventrally, is narrow and flattened 
from before backwards (cf. figs. 10 and11). In accordance with 
this the outer opening is no longer a circular blastopore, but a 
more or less crescentic slit (1. c. (30), pl. ii, figs. 31 and 32). 
In the commencement I thought it probable that the original 
blastopore of figs. 1 to 3 would become closed, and that then the 
archenteron would be pushed inwards by an epiblastic ingrowth, 
giving origin to a distinct stomodzeum, which would then be 
the anterior cavity above described. Two more considerations 
