4G0 
ADAM SEDGWICK. 
be definitely assigned either to the ectoderm or the endoderm, 
at the hind end of the blastopore. This stage, which has 
already been described in Balfour’s Memoir on Peripatus 
(this Journal, vol. xxiii), is found most commonly at about 
the middle of June in England. 
The embryo now grows considerably in length (fig. 23), the 
blastopore presenting a corresponding elongation, and the meso- 
derm, which arises from the proliferation of the undifferentiated 
cells of the primitive streak, grows forward as two ventro- 
lateral bands, one on each side of the blastopore. 
The mesodermal bands next divide by transverse division 
from before backwards into somites, which contain a cavity, 
part of the future body cavity. The first somite to appear is 
the anterior, and then successively backwards. 
Stage B (fig. 25). — The blastopore now divides into two 
parts (figs. 24 and 25) by the obliteration of its median portion 
— into an anterior part which becomes the mouth of the adult, 
and a posterior part which is at first placed at some little dis- 
tance from the hind end of the embryo and gives rise to the 
anus of the adult. 
The primitive streak still persists and extends from the hind 
end of the blastopore to the hind end of the embryo. It is 
now marked by a groove — the primitive groove (fig. 25). 
The anterior pair of somites have shifted forward to quite 
the anterior end of the body ; they give rise to the mesoderm 
and body cavity of the praeoral lobes. 
Stage C (figs. 26 and 27). — The hind end of the body now 
becomes curved ventrally (figs. 26 and 27). The curve is pro- 
duced by the growth of the hind end of the body. As this 
growth proceeds the curve becomes more marked, and assumes 
a spiral form, that is to say, the hind end of the body is spirally 
coiled, the coil being applied to the ventral face of the anterior 
part of the body (fig. 28). 
Stages B and C are found in July and August at the Cape. 
Stage D (figs. 28 and 29) . — The spiral stage is characterised by 
