METAMERIC SEGMENTATIOX AND HOMOLOGY. 
231 ) 
migration of parts after the segmentation lias been laid down 
in ontogeny, but to a redistribution at some very early stage 
in the unsegmented embryo of the formative substances. 
It is quite clear, then, that the conception of homology in a 
segmented vertebrate is independent ot any consideration of 
the number or ordinal position of the segments which compose 
the parts under comparison. If organs to be called homo- 
logous have to be composed of the same segments, we should 
have to conclude that the pectoral limbs of birds and reptiles 
are 'not homologous — which is absurd. It is merely an 
unnecessary complication to introduce the idea of segmental 
correspondence into a definition of homology. The heart, the 
liver or the lungs are held to be fully homologous throughout 
the Craniata, quite irrespective of any possible relation to 
segments ; in the same way the pectoral limb of a reptile and 
of a bird may be called homologous merely as being corre- 
sponding parts of common origin. And so it is with other 
parts of the body; the various regions of the vertebral 
column, in so far as possessed by a common ancestor, are 
homologous, but they are not necessarily composed of the 
same segments. 
Let us now consider the case of the occipital condyles,, 
which seems to present greater difficulties. There is abundant 
evidence among the Pisces that the posterior limit of the head 
is variable, that more segments are assimilated into the head 
region in some fish than in others, that by growth and 
dilfereutiation, by a process of transposition strictly com- 
parable to that of fins, the hind limit of the head moves up or 
down the segmental series. More or few’er segments become 
included in the occipital region, more or fewer gill-slits 
develop and are supplied by a corresponding number of 
branches of the compound vagus nerve. Transposition can 
account for all these changes. 
assimilation of one segment to another when it takes on the form and 
function of another. Bateson calls this liomceosis (1), but it seems to 
me difficult to apply this term to the transposition of a structure which 
occupies one segment only. 
