DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 11 



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 

 classiticaticm of the parts in relation to the sacrum, the time-honoured classification 

 of the prae-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 septum fdiaphragni). The so-called 

 " diaphragm " of birds (Huxley's " oblique septum ") is prae-cardiac, and the subdivision 

 of their coelom 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 coelomic 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 prse-sacral 

 vertebral skeleton by reference to the sternum — and we accordingly propose to substitute 

 the terms 'pr(ester7ial, sternal, and postaternal, 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 2. 



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 prsecaudal we found it always 27, viz., praesternal 8, 

 sternal 3-4, poststernal 13-14, sacral 2, with one exception for the prsesternal 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 



' E.V. Homo, juv. : Kosenberg, Morph. Jahrb. Bd. i. p. 111. 



^ On the other hand, it has been pointed out by one of us ('Nature,' vol. Ivii. p. 577) that in the JBatrachia, 

 in the absence of a costal sternum, delimitation becomes possible only in relation to the sacrum, whereupon 

 the proesacral vertebroe 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. 



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