THE TRUNK 



basis that there are no sacrals present, the length of the lumbar series be- 

 ing determined by the position of the first chevron bone of the tail. As 

 a matter of fact we have no assurance that in such a mammal without a 

 sacrum there can not have been great alteration in the chevron compli- 

 ment so that the first of these bones might now occur anterior or far 

 posterior to its ancestral position, meaning, in the latter case, that the 

 first few of the series have been eliminated. Either this has been the case, 

 lengthening of the lumbar region having been brought about by absorp- 

 tion of the anterior caudal elements, or else this has been due to the lay- 

 ing down in embryo of accessory lumbar vertebrae anlage. It is, how- 



FIGURE 30. Vertebrae of the porpoise Phocaena, illustrating differences in the 

 positions of the various details: (a and b) fourth thoracic; (c) eighth 

 thoracic; and (d and e) lumbar vertebrae. 



ever, probably not astonishing that there should be such lumbar varia- 

 tion in a mammal in which there is no attachment of the pelvis to the 

 vertebrae and in which the apaxial and hypaxial musculature have each 

 experienced such a complete degree of fusion. It seems that this would 

 involve merely a shifting forward or backward of the pelvic region by 

 shortening or lengthening of certain muscles with little regard for the 

 bones, and this should not entail any great difficulty. Presuming that 

 this has been the case, I have no idea regarding the stimuli involved or 

 the advantages gained. There is just one point that might throw some 

 light on the question. In at least most cetaceans having short or fairly 

 long (as many as about a dozen vertebrae) lumbar regions, prezygapophy- 



[179] 



