172 
Miscellaneous. 
D. While a nutritive vitellus is thus beiug formed, the exoderm, 
which seems here to act the part of blastoderm, begins to form the 
organs of the embryo. The two principal are the internal sac (for¬ 
merly “ stomach”) and the piriform organ (formerly “pharynx”); 
the former originates by the invagination of the oral surface, the 
latter by a local hypertrophy of this same surface, perhaps at the 
level of the mesodermic bands. 
E. The rest of the development is occupied by two important 
processes:—1. The growth of the crown above the aboral surface, 
dividing this surface into two distinct portions, the fold and the 
hood ; 2. The separation of the oral surface into two distinct parts, 
—that which penetrates within the crown and bears the piriform 
organ (the notched plate), and that in the centre of which the in¬ 
ternal sac opens (the rounded plate ); these are separated from each 
other by a portion of the crown, to which I give the name of the 
intermediate lobe. 
2. Metamorphosis. A. Eschaktnu; ( Lepralia ciliata). —The in¬ 
ternal sac devaginates itself and becomes transformed into a plate 
{opercular plate), tho lower surface of which serves for fixation. 
The rounded plate which covered this organ sinks down upon itself 
after its escape and becomes converted into a simple tubular body, 
which unites the inferior (oral) border of the crown to the middle of 
the upper surface of the opercular plate. At the same time the 
crown (containing the notched plate) is observed to turn suddenly 
and undergo a rotation of 90°, taking its inferior (oral) margin as 
a fixed point; its upper (aboral) margin describes a semicircle, and 
thus applies itself against the periphery of the opercular plate. In 
this movement the crown has carried with it the aboral surface, of 
which the portion folded back thus becomes visible externally, 
which from this time constitutes the whole of tho external skin, 
the hood, however, being always distinguishable. At this period 
the embryo is in the form of a cupule entirely composed of the aboral 
surface, aud having its aperture closed by the opercular plate. The 
entire crown is contained in this cupule, in the interior of which 
the vibratile cilia still project; it is contiguous to the whole inner 
surface of the cupule, and gives origin by its superior (oral) margin 
to the tubular viscus derived from the rounded plate, which traverses 
the cavity of the cupule from top to bottom. The lower surface of 
the opercular plate is destined to unite with the lower margin of 
the aboral surface to constitute the whole wall of the cell. Its 
upper surface, on the contrary, unites with the inferior (aboral) 
margin of the crown in such a manner as to form, with it and the 
central tubular viscus, a hollow ring, a torus, of the wall of which 
the notched plate which bears the piriform organ continues to form 
part. The whole of this ring is destined to degenerescence ; aud it 
is from it that is derived the thick fatty mass so often described by 
all authors ; nevertheless the notched plate and the piriform organ 
persist without undergoing this degenerescence. 
The polypide originates at this period by the invagination of the 
skiu of the hood. In this way is produced an internal sac, which is 
