366 
vestigation of the variations of the spinal nerves and vertebre in this 
genus should not have observed that the sacral diapophyses aré not 
simple. The abnormal sacra which he describes as compound and 
composed of the ninth and tenth vertebr& fused up with the urostyle, 
are, as his figures (2, Taf. 19, Figs. 3, 4 and 6) clearly show, really 
triple, and the sacral diapophyses are those of the ninth, tenth and 
eleventh vertebree. 
Throughout the whole group of the Anura the number of pre- 
sacral vertebrae, and consequently the morphological position of the 
sacrum, is remarkably constant; and the wonder is that variations 
and abnormalities are not more common. When variation in the 
number of presacral vertebre does occur, the explanation is to be 
sought not in the intercalation or excalation of vertebrae, which, as 
PARKER (33, p. 715) has already pointed out are to be looked upon 
as very rare occurrences, but rather in the shifting of the ilium for- 
wards or backwards on to the vertebra in front of or behind the 
normal. The vertebr& are from their mode of development intimately 
connected with the myotomes of the body, but the pelvis is less directly 
influenced by the primitive segmentation. It is a matter of little 
import whether it develops a little in front of or a little behind its 
normal position, and, in whatever position it develops, it seeks to 
gain attachment!) to that part of the axial skeleton which happens 
to be nearest. The vertebrae respond, and their lateral parts become 
modified accordingly in size and shape. 
In Anura, those diapophyses which, during development, happen 
to come nearest to the upper extremities of the ilia enlarge in anti- 
cipation long before they come into actual contact with the pelvis. 
The diapophyses so affected are usually those of the ninth vertebra, 
and so this has come to be regarded as the normal sacral vertebra; 
but it may be those of the eighth or the tenth, or even a combination 
of these, forming a compound sacrum. The tenth vertebra is, like 
those succeeding, only “potential”, and as a rule does not differentiate ; 
but, when from proximity of the ilium an additional strain is thrown 
upon the resources of the somite, its latent capacity for development 
is awakened, and a well formed vertebra with strong diapophyses 
results. I have seen several examples of this in Rana esculenta. 
1) It must be borne in mind that the close connection that exists 
between the ilium and the sacrum in the frog is the exception rather 
than the rule. In the majority of Anura the ilium is allowed considerable 
play, and, within limits, is free to slide forwards and backwards... This 
is more especially the case in those genera where the sacral diapophyses 
are greatly dilated. 
ee N aa 
