ON INTERCALATION OF VERTEBR^.i 



G. BAUR. 



When we have two nearly related animals, which have a 

 different number of segments, the question arises. What is the 

 origin of this difference ? There are two possibilities in regard 

 to the two forms ; one may be derived from the other, either 

 by increasing or decreasing the number of segments. In each 

 of these two cases we have different possibilities. In the case 

 of increase, the new segments may either be developed by inter- 

 calation, or by division of the original segments, or by addition 

 at the caudal end of the animal. In the case of decrease, we , 

 can think of excalation, union, or loss of segments at the 

 caudal end. 



Most morphologists are inclined to the opinion of addition 

 or subtraction of segments at the distal end. But there are 

 others, like Jhering and Albrecht, for instance, who adopt 

 intercalation. 



Let us now consider some cases. In most of the higher 

 vertebrates we have a sacrum which is united to the vertebral 

 column by sacral ribs : this sacrum establishes a more or less 

 fixed point in the vertebral axis. We distinguish presacral 

 and postsacral, or caudal vertebrae. The increased number of 

 presacral vertebrae may be produced, either by intercalation of 

 new vertebrae, or by movement of the sacrum backwards. 

 The decreased number may be the result of excalation, or 

 of the movement of the sacrum forwards. 



Cases of the movement of the sacrum have very often 

 been described, and quite a number have come under my 

 own observation. Positive cases of intercalation, however, 

 have seldom been recorded. 



Fiirbringer 2 discusses the question in the chapter, "Uber die 

 Verschiebung (Wanderung) der Extremitaeten," of his great 



1 Paper read before the American Morphological Society, Boston, Dec. 29, 1890. 



2 Fiirbringer, Max : Untersuchungen zttr Morphologic und Systematik der Voegel, 

 Amsterdam, 1888, Vol. II, pp. 972-991- 



