85 
might prove that a vertebra without any ribs is being provided 
with the latter in the course of ontogenesis, so, in view of these 
facts, a vertebral column of the formula Zf is more primitive than 
others that have fewer praesacral vertebrae and fewer dorsal vertebrae. 
Consequently we may take the vertebral column //as our starting- 
point when considering the above mentioned 10 formulas. 
If now in a vertebral column of this form the 35" vertebra is 
completely reduced, the result is a vertebral column of the formula // 
which, otherwise, with regard to the composition of the regions, 
corresponds with the vertebral column Zf. When comparing the 
illustrations, however, one can see that in the vertebral column J// 
the 13 pair of ribs consists of smaller bones and that the processus 
lateralis of the 25'* vertebra are thicker, and that they are likewise 
nearer to the pars lateralis sacri. 
These are but little differences of form, but they are forerunners 
of greater ones. 
This is already seen in the specimen, representing the formula //a. 
Here the 25th vertebra is a lumbosacral vertebra. 
This state of things becomes intelligible, when we consider, that 
the sacrum is formed, because the girdle of the lower extremities 
rests on the vertebral column and that therefore a number of vertebrae 
fuse. Further one must pay attention to the fact that the girdle of 
the extremity, (being the ossa coxae), is not connected with the 
whole extent of the pars lateralis but only with a proximal part of 
it. This fact shows, that the pars lateralis did not come into existence 
at once in its whole extent, but developed successively, and the part 
of the pars lateralis that in a given vertebral column is in connection 
with the ossa coxae, has been formed later or is younger, than the 
part lying more distally; this part was previously connected with 
these bones, but lost this connection because the girdle of the extremity 
was displaced in a proximal direction. 
At first sight this view seems to be a very hypothetical one, but 
it can be proved. 
Let us suppose “that the girdle of the extremity in a vertebral 
column of the formula // be removed only a little in a proximal 
direction, then the 25 vertebra is more strongly influenced by the 
ossa coxae. The more intense functional requirements cause a stronger 
development of the processus laterales, which can soon increase so 
much, that on one side of the body the thickened processus lateralis 
touches the pars lateralis and unites with it. In this way the 
25th vertebra can become a lumbosacral vertebra. 
This has been the case with the vertebral column //a, where the 
