No. 2.] THE STUDY (OF VARIA TION. A738 
1 example (3%) has the arch attached to the XXII vertebra. 
31 examples (88%) have “ “ se DO DO. L 
Supper sms 5 er FONT, 
Our figures show that in both normal and homeeotic speci- 
mens the point of constancy is the XXIII vertebra, and, 
moreover, that the homeceotic specimens are slightly less liable 
to vary than the normal, the former presenting 88% of cases of 
stability against 81% presented by the normal specimens. 
It would seem that the production of the first hamal arch is 
then a function of the X XIII vertebra, a function of the axial 
portion of the embryo, and that the development of the hamal 
arch proceeds entirely independent of the position of the 
neighboring appendages, for in 81% of the normal specimens 
there are three intervening vertebrz between the sacrum and 
the first hamal arch, while in nearly all of the homeeotic speci- 
mens there are but two. 
Of course if we assume that the normal position of the 
first hamal arch is on the fourth vertebra behind the sacrum, 
we can then show that the homoeotic specimens present a 
greater range of variation than normal individuals. But this 
assumption would be unjustified. 
(5) Specimens may frequently be found whose caudal verte- 
bree are provided with abnormal processes. These abnormalities 
may appear as bi-lobed or double neural plates, or double neural 
spines. Similar malformations may affect the hamal arch 
(Plate:B, Spec:70). 
If our belief in the general variability of the skeleton is to be 
justified, these minor variations should be of more frequent occur- 
rence among the homceotic than among the normal specimens. 
Among thirty-six homceotic specimens there are twenty 
which present vertebral variations in the caudal region, while 
of sixty-four normal specimens there are only twenty-six that 
show the same local variability. 
It should not be forgotten that imperfect processes of regen- 
eration may be responsible for the large number of caudal 
malformations, though this does not explain why homeeotic 
individuals should be more frequently captured during the 
regenerative process. 
