NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 267 
I regard the mouth of Amphioxus as homologous with the left half of 
the mouth of Craniota and the club-shaped gland as its antimere. That 
the mouth of Amphioxus as an organ of the left side is exactly homolo- 
gous with the left half of the mouth of Squalus appears to me probable 
on the a priori ground that it is improbable that an organ of the same 
function should be twice acquired in the Vertebrate series ; and also be- 
cause the region of fusion of endoderm and ectoderm to form the mouth 
cleft is in both these forms ventral to the constrictions which separate 
the second and third mesodermic segments (lst and 2d myotomes). 
The club-shaped gland also appears as an entodermic diverticulum below 
the constriction between the second and third mesodermic segments 
of the right side, that is, opposite the mouth diverticulum, and I there- 
fore, in agreement with van Wijhe (93), regard it as the antimeric 
gill cleft." 
In the fourth segment the following points of resemblance are to be 
noted, Somatic musculature and a somatic ventral nerve are present. 
While in Squalus the pair of visceral clefts which bounded anteriorly 
the splanchnic portion of this segment have disappeared, leaving no trace 
behind except in the neuromere with which they were connected, in 
Amphioxus only the right visceral cleft has been thus lost. The left 
visceral cleft, however, disappears ontogenetically without leaving a trace 
behind it. A further difference in the two forms appears in the fact that, 
whereas in Squal=s the dorsal nerve has disappeared (or fused with the 
trigeminus), the dorsal nerve of the left side in Amphioxus is the first 
of the nerves which innervate the musculature of the velum (van 
Wijhe). 
With the fifth segment in both forms begin the permanent visceral 
clefts. In agreement with Willey (94), I regard the first secondary cleft 
as antimeric to the second primary cleft. Their fusion with the ecto- 
derm below the mesodermic constriction between mesoderm segments 
4 and 5 (myotomes 3 and 4) is the evidence for their relation to 
this, the fifth segment. I therefore consider the first pair of permanent 
visceral clefts in Amphioxus as the exact homologues of the hyomandibu- 
lar clefts of higher Vertebrates. As has already been stated by Willey 
(94), all except eight of the primary clefts (starred in the table), which 
become paired with eight antimeric clefts, undergo atrophy. In conse- 
1 Willey (94) gives reasons for regarding the club-shaped gland as the antimere 
of the first primary visceral cleft. His reasons are based on topographic relations 
in stages when the primitive topographic relations are considerably changed, and 
they seem to me less strong than the reasons stated by van Wijhe and myself. 
