54 
The second pair of gill-slits would have atrophied, they 
are to be found again in the left and the right pseudo- 
branchial groove or “vorderer Wimperbogen” and corres- 
pond to the spiracle of fishes. The first permanent pair of 
gill-slits accordingly represents in reality the third pair. 
Recently HATSCHEK (1909, p. 508) seems to have changed 
his opinion and offers a prospect of a different interpretation 
and a homologization of both the “anterior entoderm pockets” 
and the “rostral prolongations’” to the praemandibular and 
mandibular segments, “und zwar in einer Weise, die zu der 
herrschenden Anschauung im Gegensatze steht”, Evidently 
he (1906, p. 6) now wants to consider the head cavities as 
the ventral part (hyposomite) of his anterior pair of somites 
to which the head-prolongations represent the dorsal part 
(episomite). 
Similar interpretations have been given already by VAN 
WYHE (1893, p. 157) and WILLEY (1894, p. 126). WILLEY 
compares both the anterior entoderm pockets (HATSCHEK) 
or praeoral head cavities (WILLEY) with the praemandibular 
segments of Craniates and the anterior pair of somites of 
fig. 5 with the mandibular segment of the latter. VAN 
WYHE agrees with him in the latter assumption but not 
in the former. In the two anterior entoderm pcekets he 
cannot see antimers, they are in reality median structures 
and the pocket that only secondarily, in consequence of 
the rotatory growth of the anterior part of the alimentary 
tract, has become the left one, and that passes into the 
praeoral pit, represents the old mouth of Amphioxus, the 
“autostoma”, comparable to the mouth of Craniates. Accord- 
ing to VAN WYHE the right one, passing into the praeoral 
head coelom, may be homologous to both the praemandí- 
bular cavities of Craniotes. As to the first pair of gill slits 
VAN WYHE came to very remarkable and noteworthy con- 
clusions. HIS (1887, p. 429) had observed already that the 
mouth in Amphioxus “lässt sich seiner Stellung nach eher 
einer Kiemenspalte vergleichen. An der entsprechenden 
Stelle der anderen Seite bildetsich die sogenannte kolben- 
förmige Drüse.” VAN WYHE (1893, p. 153) now found that 
the musculature and the innervation show that the mouth of 
Amphioxus is not a median structure but one that wholly 
belongs to the left side. It is situated behind the mandi- 
bular segment, as VAN WYHE calls the first segment of 
fig. 5, and is therefore compared by him to the first left 
