Charles K. Bardeen 267 



was reported sacral in natiire in 91 instances, 8.6%, and the 29th, com- 

 monly the last sacral, coccygeal in 27 instances, 2.5%. It is possible 

 that variations of this nature were sometimes overlooked by those making 

 up the tables from which the above data were obtained. 



Variation other than border variation has been reported most fre- 

 quently in the cervical region. Eibs have been found not only on the 7th 

 and 6th vertebras but also on those more anterior (Szawlowski, oi). 



It seems fair, however, to assume that the primitive vertebrae become 

 differentiated according to the demands of their environment. Thus 

 the factors commonly exerted on the 8th to 19th costo-vertebral funda- 

 ments causing them to develop into thoracic vertebrae with free ribs, 

 may be so exerted as to call into similar development the 7th to 18th, the 

 7th to 19th (20th), the 8th to the 18th, the 8th to the 20th (21st), or 

 the 9th to the 19th (20th). While the thorax may be segmentally 

 extended or reduced at either end, extended at both ends, or extended at 

 one end and reduced at the other, a simultaneous reduction at both ends 

 has not been reported (Eosenberg, 99). 



Differentiation in the post- thoracic region depends apparently in the 

 main upon the position of the posterior limb (Bardeen, 00, Ancel and 

 Sencert, 02). When the developing ilium becomes attached to the costal 

 processes of the 25th, 26th (and 27th) vertebrae the conditions of the 

 lumbar, sacral and coccygeal regions are commonly normal. But the 

 developing ilium may become attached further anterior than usual, 

 either directly to the costal process of the 24th vertebra or so far forward 

 that a close ligamentous union is established with it. In such instances 

 the 12th rib is usually either very rudimentary or absent and often the 

 29th vertebra is of the coccygeal type. In rare instances the thorax 

 may at the same time advance a segment into the cervical region. On 

 the other hand the developing ilium may take a position more posterior 

 than usual, leaving the 25th vertebra either free to develop into the 

 lumbar type or but incompletely united to the sacrum (Paterson, 93). 

 When this occurs the 20th vertebra is very apt to have ribs developed in 

 connection with it and the 30th vertebra usually becomes an integral part 

 of the sacrum. 



The coccygeal vertebra, with the exception of the first, which is more 

 directly than the others subjected to the differentiating influences of the 

 developing limb, are relatively more rudimentary in the adult than in the 

 embryo. 



Eosenberg, 76, advanced the opinion that the ilium is attached more 

 distally in the embryo than in the adult. I have recently, 1904, shown 

 that this is not the case. On the contrary, as might have been inferred 



