556 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
of the alimentary canal to a narrow slit (fig. C) and causing a par¬ 
tial collapse of the proboscis sheath and the blood vessels. The 
integument of the adult finally becomes so highly stretched that it 
is actually no thicker in many places than the integument of one of 
the embryos contained in the body. Circular and longitudinal mus¬ 
cular layers are similarly affected, so that much more than half of 
the actual cross section of the body may be occupied by the embryos, 
which are closely packed on all sides of the body. As many as 
four large embryos are sometimes encountered in a single transverse 
section (fig. C), although as a rule only two such embryos occur in 
each cross section. When naturally disposed and not too much 
crowded (pi. 23, fig. 5) the embryos occupy a single row on each 
side of the body. Although the embryo doubtless changes its posi¬ 
tion to some extent, and contracts and extends its own body at will, 
yet in most cases it assumes a fairly definite position (pi. 23, fig. 5). 
In what may be looked upon as its natural position the nearly 
mature embryo lies with its head directed toward the ventral side 
of the mother’s body. Its own body then extends dorsally to the 
lateral border of the proboscis sheath, where it is sharply bent upon 
itself until its posterior end lies beside its head and nearer the 
median line of the parent’s body. 
Some mature embryos, however, stretch out lengthwise of the 
mother’s body (pi. 23, fig. 6), and others are twisted and coiled 
irregularly, some having their heads directed dorsally. Before 
birth they commonly attain a length of from 1.5 to 2 mm. and a 
diameter of from 0.2 to 0.25 mm. 
In leaving the parent’s body the embryos rupture her body walls 
at numerous places, but the injury is in all probability insufficient to 
cause death, for the developing spermatozoa, and perhaps ova, 
remaining in her body indicate the production of a future crop of 
genital products, as stated above. 
The general course of development seems to agree rather closely 
with that of Tetrastemma vermiculus and Drepanophorus specta- 
bilis as described by Lebedinsky (’ 97 ). In numerous details, how¬ 
ever, the species at hand reveals features of the embryonic origin 
and development of certain organs of the body with remarkable 
clearness. Such of these as yield any new light on the general 
problem of nemertean development will be described in detail. 
After cleavage and gastrulation of the egg, the embryo consists 
