MUSEUM OF COMPARATIVE ZOOLOGY. 69 
two cases. In Cristatella it is long, thick, and filled with a dense mass of 
large cells (Figs. 95, cev. oœ., and 101 *, 102 *). In Plumatella (Fig. 99) 
it is very short. 
The second difference concerns the embryo itself, and is connected 
with the formation of the first polypide. In Plumatella (Fig. 99) the 
first indication of the formation of the first polypide occurs at or 
very near the neck of the oweium, or, since the ingression of cells 
into the blastocol took place at the pole of the blastula nearest the 
neck, we may say near to the pole at which ingression occurred, 
The cells of the outer layer (i.) are elongated and contain large ellip- 
soidal nuclei which are often pressed close together. All of the cells 
of the larva stain more deeply at this pole than elsewhere, and those of 
the inner layer rather more deeply than those of the outer. The nu- 
clei are also very large, those of the outer layer being possibly more 
prominent than those of the inner ; but the difference is not so marked as 
in the drawing, where too the nucleoli of the inner layer are represented 
relatively too small. Even at this stage one finds in another section 
of the same embryo the beginning of a second polypide, whose position 
is indicated at *. This second polypide is indicated merely by a con- 
siderably thickened inner larval layer, and a very slightly thickened 
outer one. The two polypides are thus seen to be wholly independent 
of each other. The first invagination further advanced is seen in cross 
section of the whole larva in Figure 96. The entire outer layer would 
seem at first sight to be involved in this invagination ; but even in this 
figure there are seen one or two nuclei which lie under the oœcium at 
the place of invagination. I believe that they will not be involved in 
it, for at a very little later stage (Fig. 104) one finds a layer of cells 
lying over the invaginated bud, which I believe are destined to form the 
ectoderm of the body wall at this place. 
Later stages in the development of the larva in this species are 
not shown. The bud follows, I am confident, the same steps that are 
pursued by the bud in the adult colony. A placenta-like connection 
of the larva with the, ocecium, which was first described by Korotneff 
(87, p. 194), begins at about this stage, and continues until two well 
formed polypides are present. This “gürtelformige Placenta” begins 
to form in about the middle of the young embryo, and the elongated 
cell of the outer layer of the larva, in contact with the oœcium shown 
on the left of Figure 99 below the *, is, I believe, the first indication 
of it. The oocium and larva both continue to increase in size, and 
the walls of the former become thinner with their increase in area. 
