DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 33 



progressive diminution in length, to the llth-12th poststernal rib. They are longest 

 throughout the middle of the series, and they arise late at Stage Q, after the ribs have 

 become chondriiied, by independent concentration of the cells which go to form the 

 intermuscular septa. They also chondrify independently at Stage E, (PI. II. lig. 7, 

 U.J).}, 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 prsesternal 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 Gunther (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 

 hsemapophysis." On the study of this he was led to regard the uncinates as 

 hsemapophj'sial 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. 



TJie 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 (PI. 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 

 o-ive 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 



' Parker, T. J. : Phil. Trans. 1891 B, p. 80. 

 ' Parker, T. J. : oj). cit. cf. pp. 87 & 113. 

 VOL. XVI. — PART I. No. 5. — Pehruary, 1901. J' 



