DEVELOPMENT OF THE SKELETON OF THE TUATARA, Ha 
and the study of its development, we desire to deal briefly with certain facts 
and matters of terminology which are necessary to render clear our method of 
procedure. Admitting for the vertebral column of all terrestrial vertebrata a primary 
classification of the parts in relation to the sacrum, the time-honoured classification 
of the pra-sacral portion of that of the Amniota into cervical, so-called ‘“ dorsal” or 
thoracic, and lumbar regions, is now wholly insufficient for purposes of accurate 
description and comparison. ‘The term ~‘ dorsal,” being one of orientation, is mis- 
applied. ‘ Thoracic,” although applicable to the middle region in the mammal—as 
embodying the region enclosed by elongated rotatable ribs and bounded posteriorly by 
the diaphragm—cannot with precision be applied to that of the lower Amniota, inasmuch 
as it presupposes the existence of a post-cardiac septuin (diaphragm). The so-called 
“diaphragm ” of birds (Huxley’s “ oblique septum”) is pre-cardiac, and the subdivision 
of their ceelom into pulmonary and cardio-abdominal compartments is in marked 
contrast to that of mammals, which is pulmo-cardiac (or thoracic) and post-cardiac (or 
abdominal). Further, when it is remembered that within the reptilian series various 
modes of ccelomic subdivision occur, and that free ribs in both birds and reptiles may 
extend beyond the limits of the pulmo-cardiac region, the application of the term 
“thoracic” to the whole of that portion of the body which bears them is apt to lead to 
confusion. And when, in addition, there are considered the presence of free ribs in 
the lumbar region of at least the young of some mammals’, the great variation of the 
ribs both in the lumbar and cervical regions of the Amniota generally, and especially 
that of the sternum in all! its relationships, simplicity and greater uniformity in our 
ideas are to be ensured by enumerating the several segments of their pre-sacral 
vertebral skeleton by reference to the sternum—and we accordingly propose to substitute 
the terms presternal, sternal, and poststernal, for the more familiar but arbitrary 
“cervical,” ‘ thoracic,” and “lumbar,” delimitating the regions by reference to the one 
organ which renders a series of terms necessary. 
Vertebral Column.—Concerning the adult vertebral column as a whole, we have not 
met with any numerical variation of its parts beyond that involving the relationships 
of the sternum duly considered in the sequel. For the caudal region the maximum 
number of vertebrae observed by us was 34 (two less than that originally recorded by 
Giinther [67. p.605]), and for the preecaudal we found it always 27, viz., presternal §, 
sternal 3-4, poststernal 13-14, sacral 2, with one exception for the presternal series. 
To an exceptional condition of the sacrum in another individual we shall return. 
During the past fifteen to sixteen years the study of the detailed constitution and early 
development of the vertebral column has received an altogether exceptional amount of 
‘ Ex. Homo, juv.: Rosenberg, Morph. Jahrb. Bd. i. p. 111. 
* On the other hand, it has been pointed out by one of us (‘ Nature,’ vol. lvii. p.577) that in the Batrachia, 
in the absence of a costal sternum, delimitation becomes possible only in relation to the sacrum, whereupon 
the presacral vertebra are best dealt with collectively. The same principle applies to those Amniota in which 
the sternum has been lost; while in the absence of a sacrum, as in the Ophidia, the course is obvious. 
(ip 
