DEVELOPMENT OF THE SKELETON OF THE TUATARA. By) 
progressive diminution in length, to the 11th-12th poststernal rib. They are longest 
throughout the middle of the series, and they arise late at Stage Q, after the ribs have 
become chondrified, by independent concentration of the cells which go to form the 
intermuscular septa. ‘They also chondrify independently at Stage R (PI. II. fig. 7, 
u.p.), at which the ribs are well ossified. Jeffery Parker has already called attention ! 
to their similar independence in Apteryx, and we accordingly follow him in the usage 
of the term uncinates. 
There can be no doubt that the process borne by the last presternal rib is a true 
uncinate, since our sections at Stages Q and R reveal its independent origin. We 
find that the “process” borne by the penultimate rib, which Giinther (67. p. 607) 
believed to be homologous with the uncinates and which Osawa definitely refers to 
them, may by elongation assume the size and shape of one in its most pronounced 
form, as contrasted with its condition described by Giinther of a mere “dilated 
heemapophysis.” On the study of this he was led to regard the uncinates as 
hzmapophysial in origin, ‘separate and removed from the distal end of the pleur- 
apophyses as the latter increase in length.” At our Stage R, at which the ribs are 
beginning to ossify and the uncinates proper are still distinct from them, the penulti- 
mate rib and its supposed uncinate are continuous, while a similar but feebly developed 
process is present on the antepenultimate rib. At the period of continuity, the 
penultimate rib and process may in some sections present appearances suggestive of a 
possible precocious union; but, inasmuch as we are unable definitely to detect 
its presence at an earlier stage or in a wholly free state, we leave it an open question 
whether the ‘‘ process ” may or may not be a mere outgrowth of the cartilaginous rib. 
If the non-extension of the osseous rib into the uncinate zone be due to loss of connec- 
tion with the sternum, the former conclusion would be the more likely; if, on the 
contrary, it be not so, it may well be that, under the extension of the unossified moiety 
into the zone, the plastic cartilage has been seized upon as available for extension and 
support under the mechanical conditions at work. 
The Sternum.—By way of descriptive detail, of the adult sternum we have nothing to 
add to the statements of Giinther and Osawa, except to state that neither has recorded 
the fact that even in the adult its posterior border is notched. At Stages R and T this 
notch (Pl. VI. figs. 5 & 6, s.n.) is very conspicuous, and inasmuch as at Stage Q the 
two halves of the sternum are widely separated, the question arises whether or not the 
notched region is one of non-union of its opposite halves. 
Parker, in describing the developing sternum of Apteryx, in the adult of which 
there are four pairs of sternal ribs, has shown ? that the two anterior of these apparently 
give rise to the sternum, and that the connection of this with the two posterior is 
secondary and due to extension of both sternum and ribs. In Sphenodon three pairs 
1 Parker, T. J.: Phil. Trans. 1891 B, p. 80. 
* Parker, T. J.: op. cit. of. pp. 87 & 118. 
VOL. XVI.—PartT I. No. 5.—February, 1901. ¥ 
