670 
The segmentation which immediately follows the protrusion of 
the polar globules is a perfectly regular one, not the slightest difference 
can be seen in the size or appearance of the segments. In this point 
Serpula differs from all Annelids with a very few exceptions (Tha- 
lassema, Sabellaria) . 
The chorion of the egg is very thick and is never thrown off but 
remains as the cuticle of the larva. There is therefore no true hatching 
of the embryo. 
As regards the relation of the blastopore and the formation of the 
mouth however, Stossich was undoubtedly in error, the conversion 
of the gastrula into the trochosphere being in the species studied at 
Beaufort essentially different from his account. The gastrula itself 
has three noticeable features. At one point is found the blastopore 
which is not round but is an elongated slit. Around the blastopore is 
a circular band of strong locomotor cilia. At the extremity opposite 
the blastopore is an ectodermal thickening accompanied by a tuft of 
long sensory cilia. ‘This thickening is the beginning of the nervous 
system (Scheitelplatte) and we know consequently that it occupies the 
anterior end of the embryo. The blastopore which is directly opposite 
would seem also to be at the posterior end. But a study of the further 
growth of the embryo shows that this is not so. When the body be- 
gins to elongate, the elongation is not in the line of the axis of the 
gastrula but obliquely to this axis in such a way as to pass through 
one end of the slit-like blastopore. Moreover this elongation chiefly 
concerns that portion of the body within the circumblastoporal ring and 
not that portion anterior to it. The result of this oblique elongation is 
that one end of the slit-like blastopore is carried backwards away from 
the other end which remains in its original position near the ciliated 
band. This causes the blastopore to become still more elongated, and 
its lips meantime close. For some time the digestive sac remains at- 
tached to the ectoderm throughout the whole course of the blastopore ; 
but after a littie it loses this connection at the middle of the long 
blastopore and only retains it at its two ends. At this time therefore 
the embryo consists of an irregular shaped body with a sensory tuft of 
cilia at one end, and with a circular band of cilia at some little dis- 
tance behind (originally the ring around the blastopore). Just poste- 
rior to this band is one end of the closed blastopore while the other 
end is at the extreme posterior of the lobe produced by the elongation 
of the body. The endoderm is a solid mass of cells united to the ecto- 
derm at two regions, one just behind the ciliated band and the other 
at the posterior end of the elongated lobe. Between these two points 
