18 PROF. G. B. HOWES AND ME. H. IT. SWINNERTON ON THE 



the sections approach the septum " ; and he adds that " the cartilage of the latter is 

 nowhere in direct contact with the cells of the chorda." The figures wliich illustrate 

 liis memoir are unfortunately diagrammatic, but that of a longitudinal section of the 

 caudal vertebra of an adult Haplodactylus (p. 26), taken conjointly with the descriptive 

 text and the associated remark that " in Phyllodactylus the chorda is only constricted," 

 leaves little doubt in our minds that he would regard the chordal plate as ectochordal. 

 We, on the other hand, are as emphatic to the contrary as concerning Sphenodon ^, 

 since we are convinced that in that animal it is endochordal and intimately associated 

 with tlie chordal epithelium in origin, as asserted by Gegenbaur. 



It first appears at Stage E (PI. II. fig. 11, n.p.) at the periphery of the chorda, and 

 entirely within the chordal sheath, which by our method of treatment stains at all 

 points light yellow as opposed to the blue of the chordal plate. On comparison of 

 Stage S with R (fig. 11), it will be seen (PI. II. fig. 12) that the plate increases by a 

 process of inward proliferation. When at S it is worked out in detail in longitudinal 

 section, two important facts are very evident, which are wholly irreconcilable with the 

 theory of an ectochordal origin — viz., in the first place, that the tissue of which the plate 

 is peripherally composed can be traced into continuity with the chordal epithelium, 

 in a manner which suggests that it arises from this by special activity of its cells ; and, 

 in the second place, that its antero-posterior faces show no sharp line of demarcation 

 from the substance of the notochord, but rather, on the contrary, a passage into it — the 

 faces of the plate being irregular, and, as it were, sending prolongations of the matrix 

 into the chorda {cf. PI. I. fig. 10). The matrix of the plate is often seen to be feebly 

 differentiated as its centre is reached, and in median longitudinal section a gradual 

 transition into the central cells of the chorda may be observed ^, 



Interesting, in consideration of recent discovery concerning the truncal origin of 

 the amniote occiput, is the fact that at this stage there is present, wholly within the 

 central tissues of the chorda (as it were in very testimony to the origin from this of the 

 plate-tissue), a similar differentiation; and the fact that the distance between it and 

 the chordal plate of the os odontoideum is about equal [cf. PI. I. fig. 9, n.p.) to that 

 between the latter and the plate next in order of succession behind, is not without 

 significance. Going back to Stage R, we find this intrachordal mass represented 

 by an essentially similar, though somewhat irregular, differentiation of the chordal 

 epithelium, over the whole length of the intracranial notochord (PI. I. fig. 9), with 



' Cf. our foobnote and reference to ZykofF, on p. 22. 



- Dr. Gadow has courteously allowed us to comiiare his sections with om own. We can confirm his discovery 

 of the ectochordal origin of the plates in Lacerta. His sections of Gecko, however, are in complete agreement 

 with ours of Sphenodon. We conclude that the plates may arise in two independent ways ; and we would 

 accordingly distinguish between the Lacertan type in which they arise by inward extension of the skeletogenous 

 tissues with accompanying constriction of the cuticular sheath (elastica) and the Gecko-Sphenodon type, in 

 which they are chordal and lie at all stages within this. A most interesting result, but by no means without 

 puiallel in the animal kingdom. 



