428 A. A. W. HUBRECHT. 
originate the cesophagus, which even in the adult animal is 
easily distinguished from the mid- and hind-gut, by its having 
no trace of lateral ceca and by the great distinctness of its 
coating of strong cilia. In Lineus and the other Schizone- 
mertea it is surrounded by the widened, lacunar portion of the 
circulatory system. 
However, we shall presently see that not the whole of the 
larval fore-gut is transformed into the cesophagus but only the 
lower part, adjoining the blastopore. We have already noticed 
that the larval fore-gut is characterised by a flattened appear- 
ance, the lumen being similarly narrowed, and in the upper 
portion often actually disappearing in the middle, and only 
remaining visible right and left of this median concrescence, 
like the two globes of a dumb-bell. This portion apparently 
becomes converted into the nephridial system, which is situated 
in the adult right and left of the esophagus in the blood lacuna 
which surrounds this. 
We will now rapidly trace the chief phases in the develop- 
ment of cesophagus and nephridial system as they were 
observed by me. The first traces of the definite cesophagus 
consist in a cell-proliferation appearing in the walls of the 
lower part of the larval fore-gut. The constituent cells 
become much smaller than they were at the commencement: 
the nuclear elements being thus more numerous in this 
lower portion it can be easily detected in stained sections by its 
more intense colouration. A layer of mesoblast cells may be 
seen to develope simultaneously and to form the enveloping 
tissue for the hypoblastic cellular layer, which is transformed 
into the cellular surface of the definite cesophagus. When a 
certain degree of development has been reached this ceso- 
phagus, arising from the walls of the larval fore-gut, secondarily 
coalesces with the cavity of the larval hind-gut, as may be 
gathered from a comparison of the figs. 9 and 13. Elsewhere 
I have given more elaborate figures of the actual sections 
from which the details of the process may further be gathered 
(1. c. (80), pl. ii, fig. 30; pl. iii, figs. 47, 48; pl. v, figs. 80, 
81, and 84). 
