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between two particular vertebrae. There is some difference in the 

 practical use of this method, some authorities meaning that in the 

 latter case a certain vertebra is wanting and (perhaps) in the former 

 that a certain vertebra is duplicated ; while others do not go into such 

 detail. If no particular vertebra be referred to this theory is not very 

 different from that of irregular segmentation. It differs, however, from 

 it in this: that inter- and ex-calation imply a correspondence 

 between certain vertebrae at the ends of the series between which 

 the change occurs, while irregular segmentation does not. 



There certainly are cases that can be accounted for only by one 

 of these methods. The following observations by Baur are very con- 

 vincing. 



All living crocodiles have 24 praesacral vertebrae, but in a certain 

 gavial he found 25, There is usually a sudden change in the trans- 

 verse process of the 12th which is the first to have the heads of the 

 ribs on the transverse processes only. In this gavial the change 

 occurred on the 13th, The 9th and 10th of the normal spine have 

 the capitellum of the rib resting on the centrum just at the level of 

 the neuto-central suture. In this gavial there were three such vertebrae. 

 Hence Baur concludes that there is an intercalation between the 9th 

 and 10th. It would be better, in my opinion, to say that in the place 

 of two vertebrae there are three, and not to attempt any more precise 

 homology, I agree with Bateson that ''individuality should not be 

 attributed to a member of a series which has normally a definite 

 number of members". Mention should here be made of the celebrated 

 python in the Museum of the Royal College of Surgeons, discussed 

 lioth by Baur and Bateson, in which the 166th and the 185th vertebrae 

 are double on one side, with each two transverse processes and two 

 ribs, while they are single on the other. What has occurred is a 

 unilateral error of segmentation, explain it as one may. 



It is self evident that the intercalation theory, or the better one 

 of an error of segmentation, in no way excludes Rosenberg's, The 

 migration of a developing ilium is not in the least inconsistent with 

 a cervical vertebra too few or too many, depending on other causes. 



The vertebra fulcralis. There is one important point in 

 the discussion of the rival theories, which was raised long ago by 

 Welcker, and which needs to be settled. According to what principle 

 are the vertebrae of one spine to be compared with those of another? 

 Do, for instance, the 19th and 24th vertebrae of one spine correspond 

 to those of the same number in all others ; or does the last praesacral 

 always correspond to the last praesacral, though it be the 23d, the 



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