Plate 1. 
Diagrams of the head of the main groups of Chorda- 
tes; the black dots indicate the hypoglussus-roots. 
Ac. cranio-vertebral limit in Acanthias 
a. p. animal pole 
can. med. medullary tube 
Ct. cranio-vertebral limit 
EH. groove of HATSCHEK 
m. mout 
m. tymp. tympanic membrane 
neut. Q. neural arch 
np. neuropore 
Oc. eye 
OCC. Q. occipital arch 
olf. olfactory groove 
pt. mand. prae-mandibular cavit 
SC. cranio-vertebral limit in Scyllium 
sp. spiracle 
stom. ‘stomodaeum 
1, 2, 3, 
(According to VAN S 
somites 
ETERS’s recent investigation (1921) 
the scheme given here for Amphibia does not apply to 
Anura where the cranium is one segment shorter and the 
first myotome belongs to the sixth segment). 
entoderm 
Li Amphibia. 
(Nectu rus) 
gn | | (Seyllium 
| Acanthias) 
cr. 
1 2 3 4 5 6 7 8 | 9 10 | 12 13 14 
EO 
Te Ree rj Bd dee JMie a ad = 
6 Ke 
le) fl X | 
| | Amniotes. 
de tymp (Lacerta) 
Î 
fj 
