DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 13 



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 Gadovv 

 [nc.s., PI. 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) difi'erentiated within the area of this more closely 

 aggregated stratum a series of metamerically recurrent masses {i.p.) of cells, generally 

 more rounded than the rest. They are the developing intercentra ; wherefore itfolloAvs 

 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 sheatlt, 

 the aggregation afore-named being apparent {i-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, «.«.), 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 (t.a., text-fig. 2 and PI. 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 {n.a.) and confined to the superficial layers of the 



