390 
The relation of dorsal nerve roots to VAN WIJHE’s 2"4 to 6th somites 
has been stated. I regard as the ventral motor roots of these 
segments the trochlearis and the abducens. The trochlearis is the 
ventral motor root of the third encephalomere and van W1JHE’s 
second somite. The abducens represents the motor nerves which 
primitively innervated somites 3, 4, 5 and 6. The facts which lead 
me to so regard it are as follows; (1) The wide separation and 
multiplicity of its roots; (2) The distribution of its fibres in early 
stages to mesoderm posterior to the third somite, which it finally 
innervates in 8. acanthias; (3) The variable position of the exit of 
its fibres: from encephalomere VI (chick, swine and? Necturus) and 
VII (S.. acanthias); (4) its elongated nucleus in the anterior 
horn; finally (5) the fact that in Petromyzon the muscle which it 
innervates (posterior rectus) extends forward from the myoseptum 
between the first and second occipital somites (homologous with 
VAN WIJHE’s 5th and 6th) and is thus formed, at least in part, as 
stated by Kuprrer (94), from a post-otic myotome. The position of 
the exit of its fibres in S. acanthias shows it to be in that form a 
post-otic nerve. The distribution of a post-otic ventral root, which is 
homologous with ventral spinal roots, to pre-otic musculature is an- 
other link in the chain of evidence showing that no fundamental 
distinction between pre-otic and post-otic regions exists. 
With this evidence of the metameric value of the hindbrain 
encephalomeres (IJI—VII), may we not accept the anterior forebrain 
and midbrain encephalomeres (I and II) as criteria of the existence 
of metameres in the most highly differentiated region of the Verte- 
brate body? The loss of motor nerves in relation with encepholomere 
I is to be explained by the loss of the musculature of that segment. 
The relation of the ventral motor root, the oculomotorius, and 
VAN WIJHE’s first somite to the primary midbrain expansion (en- 
cephalomere II) justifies the opinion that these are components of 
a single metamere only. 
The most important conclusion which I have reached in my 
Studies seems to me to be that there is a serial homology between 
pre-otic and post-otic segments in the Vertebrate head. This con- 
clusion rests upon ths following facts; (1) an unbroken continuity of 
post-otic and pre-otic neuromeres (encephalomeres); their exact 
similarity on the grounds of relations with nerves, osmites and vis- 
ceral arches; and (3) the distribution of post-otic (Somatic) fibres to 
pre-otic musculature. 
