Eimbryogeny of the Bryozoa. 273 
-We see therefore that the sinking of the oral surface into 
the interior of the embryo must here be preceded by a very 
important phenomenon, the reversal of the mantle. The larva 
suddenly quits the special arrangement which it affected 
among the Escharina, to revert to a form nearer that of the 
Entoprocta, and in which the aboral surface may again close 
over all the rest. 
When the reversal of the mantle is effected the pallial 
cavity has completely disappeared; the larva then consists 
only of a simple sac entirely formed by the aboral surface, 
which has contracted around the edges of the adhesive lamina ; 
in the interior is the cavity of the vestibule, margined by the 
circlet and the oral surface, the latter returning upon itself 
towards the centré, to be continued into a small tube which 
traverses the cavity of the vestibule and unites with the ad- 
hesive lamina. 
3. Lormation of the polypide and of the opaque globules.— 
The entire wall of the vestibule (including the circlet), the 
oral surface (including the prebuccal organ), and the upper 
part of the adhesive lamina are destined to fall into degene- 
rescence, to form the thick mass of opaque globules which 
afterwards lines the whole base of the ceil. The two small 
thickenings in the form of ridges situated towards the top of 
the vestibule in this stage alone escape the process of degene- 
rescence ; they increase while all the rest begins to be atro- 
phied, and finally unite above the vestibule to form a single 
mass of increasing volume, and which afterwards passes 
towards the superior and anterior part of the future cell. At 
this point it meets with a second rudiment, originating from 
the invagination of the hood, and becomes confounded with it 
to form the polypide. In Lepralia ciliata the invagination 
of the hood gives origin to the whole of the internal epithelial 
layer of the polypide; the rudiment originating from the 
wall of the vestibule furnishes all that belongs to the external 
muscular layer. 
It seems to me legitimate to see, in the two parts of the ves- 
tibule of the larvee of Hscharina’ (the rudiment which is de- 
tached from the upper portion, and the remainder of the wall 
destined to fall into degenerescence), parts corresponding to 
the two great divisions, upper and lower, of the vestibule of 
the Entoprocta, of which the former likewise forms the poly-, 
pide, while the second breaks up into globules exactly com- 
parable to the mass of globules of the young cells of. the 
Hscharina. 
As to the invagination of the hood, I regard that as homo- 
