266 Studies of the Development of the Pluman Skeleton 



mine. While it is unlikely that experimental studies of the required 

 nature can ever be made on mammalian embryos it is quite possible that 

 they may on embryos of some of the lower vertebrates. From the evi- 

 dence at hand it seems probable, however, that the primitive vertebrse are 

 to a considerable extent potentially equivalent and that their subsequent 

 development depends upon the demands of their regional environment. 

 The strongest arguments in favor of this view come from a study of 

 variation in the adult. It is well known that at the regional boundaries 

 vertebral variation is frequent. Thus the 7th vertebra often carries a 

 short "cervical" rib (Gruber, 6g), and rarely it has two cervical ribs 

 which run to the sternum (Bolk, oi). On the other hand the 8th ver- 

 tebra, usually the first thoracic, may assume all the characteristics of a 

 cervical vertebra (Leboucq, 98, Low, 01). 



At the thoracico-lumbar margin variation is more frequent than at the 

 cervico-thoracic. Thus out of 1059 instances described, statistically in 

 the literature I found, 1904, that the 19th vertebra, commonly the last 

 thoracic, had no free ribs and was hence of the lumbar type in 30 

 instances, 2.8%, and that on the other hand the 20th vertebra, commonly 

 the first lumbar, had free costal processes in 23 instances, 2.2%. Cases 

 have also been reported where the 21st vertebra, usually the 2d lumbar, 

 has carried free ribs (Rosenberg, 99). Variation takes place in the 

 articular processes as well as in the costal elements of the vertebra; at the 

 thoracico-lumbar border (Topinard, 77). 



Variation of the lumbo-sacral boundary is likewise frequent. Thus out 

 of the 1059 instances above mentioned in 28 instances, 2.7%, the 24th 

 vertebra, commonly the 5th lumbar, was the first sacral and in 47 

 instances, 4.4%, the 25th vertebra was of the lumbar type. The 25th 

 vertebra may exhibit one of many transitions from the sacral to the 

 lumbar type. This subject has been well treated by Paterson, 93. 

 Papillault, 00, has contributed an interesting paper on lumbar variations 

 and Cunningham, 89, on the proportion of bone and cartilage in the 

 lumbar region. 



At the sacro-coccygeal border variation is even more frequent than in 

 the regions more anterior. Paterson, 93, found diminution in the number 

 of sacral vertebrse in 2.62%, and increase in their number in 35.46% of 

 the 265 sacra he examined; and Bianchi, 95, in 17.5% of the female, 

 23.3% of the male, and 21.23% of the total number (146) of sacra 

 examined. In this count he excluded sacra in which compensation for 

 lumbo-sacral alterations was to be seen. Bianchi thinks that the 1st 

 coccygeal vertebra belongs properly to the sacrum. In the 1059 instances 

 mentioned above I find that the 30th vertebra, usually the 1st coccygeal. 



