212 
If we compare the embryo of fig. 5 and 6 with an out- 
growing trochophora which has already produced a number 
of segments, the episphere is the part lying in front of the 
first mesodermic segment and the outgrowing soma is the rest 
of the body. A difference is that the ectomesoblast seems 
to have totally disappeared. 
econdary mesoblast in the prostomium. — In Annelids not 
only the original mesenchyme of ectodermal origin is found 
in the prostomium but also the 
coelomic trunk mesoderm sends 
out into itsecondary prolongations. 
(1886, p. 11) and KLEINENBERG 
(1886, p. 148) already, and MEYER 
(1890, p. 299) remarks: “Bei den 
Anneliden besitzt der Kopflappen 
lraina airranan 
sondern erhält seine peritoneale 
Auskleidung, wie ich mich überall 
davon überzeugt habe, durc 
Ausdehnung der Wandungen des 
ersten postoralen, also Rumpfso- 
mitenpaares nach vorn, wodurch 
die primäre Kopfhöhle vollständig 
verdrängt wird.” This latter state- 
ment, according to EISIG (1899, 
p. 230), is not right; in front of Ei 
the brain, which remains con- darten beden B 
nected with the ectoderm, the (after SHE nie Ind 1882, fig 47). 
coelomesoblast cannot penetrate. 
Thus the antecerebral part of the prostomial cavity preserves 
its blastocoelic nature and pe muscle fibres extended in it 
are of ectomesoblastic ori 
Just as in Annelids, He “foremost pair of somites in 
Amphioxus secondarily provides the prostomium with meso- 
derm, each sending out a forward prolongation into it, 
known as the rostral or head-prolongation (Kopffortsatz). 
The anterior end of the notochord in fig. 5 exactly corres- 
ponds to that of the series of somites; it indicates the 
limit of prostomium and soma and accordingly lies right 
under the neurope. Afterwards, however, the notochord, 
together with the rostral prolongations of the first pair of 
somites, grows out into the prostomium, providing a firm 
