214 TEXT-BOOK OF EMBRYOLOGY. 



six. Occasionally a vertebra between the lumbar region and sacral region — 

 lumbo-sacral vertebra — possesses both lumbar and sacral characters, one 

 side being fused with the sacrum, the other side having a free transverse process. 

 Variation occurs frequently in the coccygeal vertebra;; four and five are present 

 with about equal frequency, more rarely there are only three. 



The total number of true (presacral) vertebrae may be diminished by one or 

 increased by one. In the former case the first sacral is the twenty-fourth ver- 

 tebra, and, if the number of ribs remains normal, there are only four lumbar 

 vertebras. In case the total number is increased by one, the first sacral is the 

 twenty-sixth vertebra, and there are twelve thoracic and six lumbar or thirteen 

 thoracic and five lumbar. 



From these facts it is seen that variation occurs most frequently in the more 

 caudal portion of the vertebral column — in the lumbar, sacral and coccygeal 

 regions. According to a hypothesis advanced by Rosenberg, the sacrum in the 

 earlier embryonic stages is composed of a more caudal set of vertebras than those 

 which belong to it in the adult, and during development lumbar vertebras are 

 converted into sacral and sacral vertebras into coccygeal. In other words, the 

 hip bone moves headward during development and finally becomes attached to 

 vertebras which are situated more cranially than those with which it was pri- 

 marily associated. This change in the position of the pelvic attachment, and the 

 corresponding reduction in the total number of vertebras, during the develop- 

 ment of the individual (i.e., during ontogenetic development) is believed to 

 correspond to a similar change in position during the evolution of the race (i.e., 

 during phylogenetic development). 



According to Rosenberg, variation in the adult is due largely to a failure 

 during ontogeny to carry the processes of reduction in the number of vertebrae 

 as far as they are usually carried in the race, or to their being carried beyond this 

 point. 



The coccygeal vertebras apparently represent remnants of the more exten- 

 sively developed caudal vertebrae in lower forms. In human embryos of 8 to 

 1 6 mm., when the caudal appendage is at the height of its development, there 

 are usually seven anlagen of coccygeal vertebras. During later development this 

 number becomes reduced by fusion of the more distally situated anlagen to the 

 smaller number in the adult. This process of reduction varies in different in- 

 dividuals, so that five or four, rarely three, coccygeal vertebras may be the result. 

 In cases where children are born with distinct caudal appendages there is no 

 good evidence that the number of coccygeal vertebras is increased, although the 

 coccyx may extend into the appendage. 



The Ribs. — Occasionally in the adult a rib is present on one side or on 

 each side in connection with the seventh cervical vertebra (cervical rib), or in 

 connection with the first lumbar vertebra (lumbar rib). There seems to be no 



