WAITE : PLEXI OF NECTURUS. 83 
initial variation in the serial number and position of centres of metamer- 
ism, — a doctrine formulated by Bateson (792, p. 111); and, secondly, 
that such change arises from fusion of two or more adjacent somites, or 
from the splitting of one or more somites somewhere in the presacral re- 
gion. The first of these two views is the more acceptable as a morphologi- 
cal process, as it preserves the integrity of the metameres and makes the 
process a general one affecting the animal as a whole, rather than local- 
izing the activity within narrow limits, but the evidence in its favor is 
mostly a priori. 
Rosenberg (76, p. 104) was one of the first to advance such a view, but 
he went further than is implied in the above statement, and held that 
this process is an actively phylogenetic, one, and that in the vertebrate 
series'its operation results in a constant shortening of the vertebral col- 
umn. This shortening is greatly emphasized in some orders, e. g. 
Anura, whereas such conditions as occur in Ophidia must, on the con- 
trary, be considered reversionary. 
According to his view, in the early ontogeny there is represented the 
primitive condition with a very large number of separate prosegments, 
but in course of development of the individual these are reduced to the 
number set by heredity. We may infer variation in the final number 
of segments to be the result of inaccuracy of response to the hereditary 
stimulus. As noticed by many writers upon this subject, the great 
majority of such variations are toward an increase in the number of 
segments, and according to this view such cases must be interpreted as 
atavistic. It would seem that Rosenberg’s view is insufficient, or at 
least if there be such a general tendency, it is exerted, not in a single 
unilateral series, but in a branching series of phylogenetic relationships. 
In such cases of variation between two individuals either of the same 
or of different species, no one centre of the final series of metameres in 
the normal can be directly homologous with any one centre in the vari- 
ant; i.e. the number of segments arising in the variant differs from that 
arising in the normal, and each segment occupies a new position different 
from that in the normal, : 
Most of the evidence from recorded skeletal abnormalities better fits 
the second category stated. Baur (91, p. 335) claims intercalation of 
vertebre as an actual ontogenetic process. This is based upon the evi- 
dence afforded by certain complexes of vertebrae which show incomplete 
fission on one side of the median plane, with a normal condition on the 
other side, these having been recorded in some Ophidia by Albrecht, 
Owen, and Baur himself. Other cases of vertebral “ masses ” have been 
