DEVELOPMENT OF THE SKELETON OF THE TUATARA. 1s: 
Certain it is that this subject, as it now stands, forms one of the most fascinating 
chapters in the vertebrate morphology of our time. Our own researches begin at the 
period of differentiation of actual skeletogenous tissue, and while, concerning the 
supposed original complexity and general uniformity in origin of the parts of the 
vertebral body, we retain an open mind, we question if, in matters of detail to be 
referred to in the sequel, theories, in their growth, have not been at times unconsciously 
read into the supposed record of facts. 
First differentiation of the Skeletogenous Tissue.—The earliest stage which we have 
investigated is Dendy’s P, and a median longitudinal section through any portion of the 
skeletal axis at this stage reveals the presence of a central notochord, ensheathed in a 
thin structureless and deeply staining cuticle or chordal sheath (elastica externa of Gadow 
[nc.s., Pl. I. fig. 2]). This is seen to be in turn invested by a continuous skeletogenous 
sheath, composed of a thickly disposed mass of small cells, containing relatively large 
nuclei. There are some 9-10 rows of these, and their close compression gives to the 
tissue, when examined under a low magnifying power, the appearance of being feebly 
longitudinally striate. ‘The notochord itself consists at this stage of comparatively 
small vacuolated cells, arranged in an irregular manner, and differentiated peripherally 
into the usual chordal epithelium (nc.e.). Central or other differentiation there is 
none. 
Comparing with the foregoing a lateral longitudinal section, the superficial cell- 
rows of the skeletogenous layer are seen to be more closely approximated than the rest, 
and in addition there are (fig. 1) differentiated within the area of this more closely 
aggregated stratum a series of metamerically recurrent masses (¢.p.) of cells, generally 
more rounded than the rest. They are the developing intercentra ; wherefore it follows 
that in order of differentiation these are the first formed skeletal elements. 
When the corresponding parts are viewed in transverse section, the skeletogenous 
layer is seen to completely encircle the notochord, constituting a skeletogenous sheath, 
the aggregation afore-named being apparent (7.p.) as a couple of symmetrical tracts 
which are ventro-lateral, widely separated in the trunk-region (cf. fig. 2), closely 
approximated in the ventral line in the caudal (fig. 3). The neural arches are at this 
stage first differentiated, in the form of bilaterally symmetrical upgrowths of the 
skeletogenous sheath (fig. 3, 7.a.), separated by a wide interval in the mid-dorsal line. 
Stage Q.—A transverse section through an intervertebral region at this stage shows 
that, concerning the skeletogenous sheath (¢.a., text-fig. 2 and Pl. I. fig. 6), the cells 
remain practically unchanged, their nuclei when examined under a high power presenting 
the oval contour indicative of mutual apposition, so characteristic of the earlier stage 
afore-described. 
Comparison of a similar section through a vertebral region shows (text-fig. 3) that 
chondrification has there set in, but not at all points—for, while laterally hyaline 
cartilage is now present, in the form of paired masses (c.v.), continuous with the 
now cartilaginous neural arches (.@.) and confined to the superficial layers of the 
