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 intercalation, 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.
I n 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 vertebra The increased number of presacral vertebrae may be produced, either by intercalation of new vertebrze, 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.
Furbringera discusses the question in the chapter, "Uber die Verschiebung (Wanderung) der Extremitaeten," of his great Let us now consider some cases.