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SIDNEY F. HARMER. 
characters retained in all subsequent stages ; throughout life it 
consists of a single row of cells bearing long cilia. 
In a still later stage (fig. 45), the two vestibular cavities ( v .) 
have fused in the middle line, and form a transversely extended 
groove separating from one another the process of the body 
bearing the rectum ( rec .) and the epistome ; the mouth at this, 
as at other stages of the embryonic history, is a transversely 
elongated funnel, connected on each side with a groove 
which runs round the ventral surface of the body, just 
within the ciliary ring. This “ oral groove ” is represented 
in fig. 40, og. 
Only the deeper portions of the vestibule are formed from 
the invaginations just described; the peripheral parts originate 
from a growth ventralwards of the region of the body bearing 
the ciliated ring, which can be either extended to the exterior, 
as in swimming, or can be retracted into the interior of the 
vestibule, whose aperture is then constricted, as indicated in 
fig. 55, va. 
We may now take up the further history of the dorsal organ 
or brain, which we left in the condition of a thickened area 
of the epiblast, composed merely of a single layer of cells. 
In fig. 38 the dorsal organ (br.) is seen to have become two 
cells thick. Fig. 46 represents a somewhat earlier stage, seen 
in horizontal section ; the epiblast, elsewhere very thin, has 
proliferated off a bilobed cellular mass, the wings of which 
pass as far as the sides of the oesophagus. The number of 
cells composing this structure, the brain, increases consi- 
derably, and before long it may easily be observed by means of 
horizontal sections (figs. 47 and 48) that the cells are arranged 
in a single layer round a lumen which opens medianly to the 
exterior. It is probable that the brain is at first formed by a 
solid ingrowth of cells, in which a cavity subsequently appears, 
although it is not impossible that a lumen may be present from 
the very commencement, and that owing to its small size it 
cannot easily be detected in sections. In either case, the brain 
is formed by a process of epiblastic invagination. 
Hatschek (14) has already illustrated and described in 
