No. i.] INTERCALATION OF VERTEBRAE. 53 



Ridewood has discussed fully all the known cases of varia- 

 tion in the number of presacral vertebrae in the Ecaudata. He 

 makes the following remarks (p. 366): "Throughout the whole 

 group of the Anura (Ecaudata) the number of presacral verte- 

 brae, and consequently the morphological position of the sacrum, 

 is remarkably constant ; and the wonder is that variations and 

 abnormalities are not more common. When variation in the 

 number of presacral vertebrae does occur, the explanation is to 

 be sought, not in the intercalation or excalation of vertebrae, 

 which, as Parker ('96) has already pointed out, are to be looked 

 upon as very rare occurrences, but rather in the shifting of the 

 ilium forwards or backwards on to the vertebra in front of or 

 behind the normal. The vertebrae are from their mode of 

 development intimately connected with the myotomes of the 

 body, but the pelvis is less directly influenced by the primitive 

 segmentation. It is a matter of little import whether it de- 

 velops a little in front of or a little behind its normal position, 

 and, in whatever position it develops, it seeks to gain attach- 

 ment to that part of the axial skeleton which happens to be 

 nearest. The vertebrae respond, and their lateral parts become 

 modified accordingly in size and shape. 



" In Anura (Ecaudata) those diapophyses which, during devel- 

 opment, happen to come nearest to the upper extremities of 

 the ilia enlarge in anticipation long before they come into 

 actual contact with the pelvis. The diapophyses so affected 

 are usually those of the ninth vertebra, and so this has come 

 to be regarded as the normal sacral vertebra; but it may be 

 those of the eighth or the tenth, or even a combination of these, 

 forming a compound sacrum. The tenth vertebra -is, like 

 those succeeding, only ' potential,' and as a rule does not dif- 

 ferentiate; but when from proximity of the ilium an additional 

 strain is thrown upon the resources of the somite, its latent 

 capacity for development is awakened, and a well-formed verte- 

 bra with strong diapophyses results. 



" As soon as n<e admit that, in Anura at least, any vertebra can 

 become sacral, and that it only requires the stimulating presence 

 of the iliac cartilages to induce an exaggerated development of 

 the diapophyses, all the mystery of abnormal sacra is dispelled, 



