562 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
At this time the space between the dissepiments is filled with a solid 
mass of cells. The cells then split apart to form a lumen contin¬ 
uous with that of the main intestinal canal. The cells of the pouch 
are then rearranged into a single layer on each side of the lumen, 
each cell assuming a more columnar shape. In the elongation of 
the cell to its final columnar form the cytoplasm and vacuoles are 
rearranged in the cell, and the nucleus is pushed down toward the 
base, that is, toward the side of the cell farthest from the lumen. 
The endodermic cells of the early embryo are of large size and 
comparatively few in number. Each cell is thickly crowded with 
yolk spheres, most of which are rapidly absorbed, although a few 
remain until the embryo is nearly mature. With the absorption of 
the yolk occurs a marked vacuolation of the cytoplasm (pi. 24, figs. 
12, 16), which increases in prominence up to the time of the forma¬ 
tion of the definite epithelium, when the cell changes its shape as 
above described. With the assumption of the ultimate columnar 
form of the cell, the vacuoles disappear, and are replaced by the 
granules of the secretion so characteristic of the intestinal cells of 
the adult. With the growth of the body the central lumen of the 
intestine becomes comparatively narrower, and the intestinal lobes 
very much deeper; this applies with equal force to the intestinal 
caecum, which has similar paired diverticula. 
The anus and rectum are formed, as in other species, by mutual 
ingrowth from the ectoderm at the posterior end of the body and 
outgrowth of the posterior end of the intestinal canal. The alimen¬ 
tary canal thereby assumes the form of a continuous tube open at 
both ends of the body, with intestinal diverticula as in the adult. 
The lining of the rhynchodaeum, esophagus, stomach, pylorus, and 
anus is thus derived from the ectoderm, while the lining of the intes¬ 
tine proper and intestinal caecum is endodermic in origin, as stated 
above. This appears to be true not only for the Hoplonemertea 
in general, where a direct type of development is the rule, but 
also for the Heteronemertea, which pass through a complicated 
metamorphosis. 
Proboscis .— The ectodermic invagination forming the rhvncho- 
daeum widens out into a conspicuous chamber immediately in front 
of the brain, from which the esophagus grows back beneath the 
brain commissure as described above. A similar posterior growth 
from this chamber passes between the brain lobes on the dorsal 
