WAITE : PLEXI OF NECTURUS. 83 



initial variation in the serial number and position of centres of metamer- 

 ism, — a doctrine formulated by Batesou ('92, p. Ill); 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 veiy 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 Iiereditary 

 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 Eosenberg'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 iu 

 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 anew 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 

 vertebra) 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 



