DEVELOPMENT OF THE SKELETON OF THE TUATARA. 35 
is considerable variation. Boulenger has called attention! to the fact that “the 
anterior extremity of the plastron overlaps the posterior extremity of the sternum,” and 
concerning this, two specimens we have examined yield interesting results. In each of 
them there are two free gastralia in front of the first that is attached to the ribs; and 
while in one example (that possessed of four pairs of sternal ribs) these are short and 
median, in the other (that in which the last “cervical” rib is sternal) the anterior 
gastralium is represented by a symmetrically-arranged pair of rods. 
Giinther originally described each gastralium as of three elements, and their natural 
relations are generally those indicated in Pl. II. fig. 10, no.7. Not unfrequently union 
in the middle line, as figured and described by him, may occur, and it would seem to 
involve a forwardly-directed spur on the median element which is generally present 
(cf. Pl. Il. fig. 10, no. 11). Much interest attaches to Giinther’s description of an 
exceptional specimen, in which for the rib-connected gastralia the lateral elements were 
attached to the central by transverse “ joints,” instead of being in oblique juxtaposition 
with them as for the free gastralia and as is more generally the case throughout the 
entire series. The fact that, in most specimens we have examined, individual gastralia 
of both the attached and free order show both “ jointing” and juxtaposition, and that 
while in most cases one or two such “joints” may be present only on one side—there 
being no regularity recognizable,—proves that no physiological significance is to be 
attached to these variations. 
Turning to the embryo, we find that the gastralia arise at Stage S, within previously 
differentiated fibrous tracts of the subcutaneous tissues of the body-wall (g.¢., Pl. I. 
fig. 9), by a process of calcareous deposition, which proves them to be wholly 
membranous in origin. ‘Their connection with the myocommata of the body-wall is 
fully established later, and with Gadow? we have failed to detect any traces of a 
cartilaginous origin, believed by Schneider to be the case for the Crocodilia. Figs. 8 
and 9 represent (8) the anterior (third to fifth), 9 the posterior (16th to 18th and more 
highly magnified) gastralia of an example at Stage S. It will be observed that the 
median element is present only anteriorly, and that while for two of the three segments 
figured comparison with the adult shows its lateral cornua to be partly represented by 
a series of minute calcifications, for the anterior segment it is composed of two pieces. 
Anteriorly the lateral elements are seen to be fully formed, but on passing to the posterior 
series two of the three figured are observed to be represented by a linear series of 
distinct calcifications, the median element being undeveloped. The fact that in the 
adult the gastralia which occupy the positions of those here figured are alike 
symmetrical and only trisegmental, proves that with advancing development a union of 
the calcifications such as are here figured must occur, and that the median elements 
’ Boulenger, G. A.: Brit. Mus. Cat. Chelonia, Rhynchoceph. & Emydos. 1889, p. 1. 
* Gadow, H.: Morphol. Jahrb. Bd. vii. 1882, p. 77. 
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