DEVELOPMENT OF THE SKELETON OF THE TUATARA. 17 
common to the whole sheath; but there has appeared around them a feebly hyaline 
matrix. ‘lhe now clearly recognizable differences in the parts generally admit of the 
application of the term interarticular tissue to these intervertebral areas ; and we desire 
to emphasize the fact that though they are feebly hyaline, “ intervertebral disks ” are not 
present—the hyaline matrix, dense and resistant for the vertebral bodies, diminishes 
markedly as the extremities of the centra are reached, and at these fades into the mure 
flexible interarticular tissue, there being no hard and sharp line between the two. 
The process of differentiation at this stage is most noteworthy for the full formation 
of an important constituent of the individual vertebra, which it so happens our 
reagents differentiate with startling effect. This, which we propose to term the chordal 
plate, is located transversely in the central and most constricted part of the developing 
vertebra. We have encountered it (Pl. I. fig. 10, n.p.) in all regions of the column, 
from the first (odontoid) vertebra (fig. 9) to well into the region of the tail. Micro- 
chemically, it reacts in a totally different manner to the reagents we have employed to 
anything else thus far described, wherefore there is no room for ambiguity concerning 
it. Its general matrix, which is extensive, stains a deeper blue than any other form of 
skeletogenous tissue which is present, and when examined under a high power its very 
conspicuous appearance is found to be due to the extent of its matrix and the presence 
of regular circular spaces, each filled by a cell which hardly stains at all and contains 
a conspicuous rounded nucleus. From its general appearance and disposition we 
have no doubt that it represents the so-called chordal cartilage described by Gegenbaur 
in 18621 in the Urodela and Reptilia, and since observed by Field, Fraser, and Stohr 
in certain allied forms and by Goette in the Anura. 
Gadow has given (op. cit.) an admirable summary of the work of these investigators, 
all of whom regard this as chordal in origin, and of others who seek to disprove that. 
With the latter he is himself agreed. He deals with the question in full in Lacerta 
and the Geckos among Lacertilia, and since we are dealing with Sphenodon, it is with 
his statements that we are alone concerned in detail. Having referred (op. cit. p. 11) 
to an “occasional perforation of the elastica externa” in the Urodela, he is most 
emphatic as to the rending asunder and destruction of the chorda (p. 26) during the 
formation of the plate in certain Lacertilia. He describes this plate as a septum 
of which the cells “retain the appearance of young or embryonic cartilage.” Dealing 
with the embryonic Lacerta, he asserts (p. 24) that he is ‘“ absolutely certain this so- 
called chordal cartilage..... does not exist, if thereby be understood cartilage which 
is produced by the conversion of chorda cells; nor does this cartilage invade the 
chorda.” His context shows this statement to be based mainly upon the examination 
of transverse sections, of which he says that they ‘“‘allow the elastica externa to be 
traced as a continuous, unbroken line, which becomes irregularly folded, or creased, as 
1 Gegenbaur, C.: Untersuchg. z. vergleichd, Anat. d. Wirbelsiiule b. Amphib. und Rept. 4to. Leipzig, 
1862. 
VOL. XVI.—PaRrT I. No. 3.—February, 1901. D 
