16 PROF. G. B. HOWES AND MR. H. H. SWINNERTON ON THE 



are still more closely aggregated than those of the centra, wherefore they appear under 

 the microscope dark and highly conspicuous. 



Stage E. This stage is most noteworthy for the completion of chondrification in 

 the median plane (cf. PL II. fig. 11) of the vertebral bodies, throughout the whole 

 length of the column, and for the lateral extension of these. Ossification of them has 

 now set in, chiefly in the dorsal and ventral surfaces as seen in transverse section 

 (PI. II. fig. 11, o.v.). The neural arches begin to ossify independently of each other 

 and the centrum, by an identical and similarly superficial process, as represented (for 

 Stage S) at PL II. figs. 4-6. 



At this stage a remarkable change, for which we were not prepared, is undergone 

 by the intercentra of the segments numbering 11 to 29 (i. e. those between and 

 including the third sternal and second caudal segments), viz., the complete disappearance 

 of those belonging to segments 13 to 25 (cf. table, column R, p. 27) ; and the tendency 

 to disappearance, to an extent which renders it difficult to identify their presence, 

 for segments 11 and 12 and 26 to 29. We are unable to say at this stage by what 

 process this removal is effected, whether by absorption into the intervertebral tissues 

 or otherwise ; but, in view of the discovery of this remarkable fact, it is the more 

 astonishing to note that while one pair (viz. those of the third caudal segment) 

 remain in their primitive condition, those which follow this in order of succession 

 behind, while still unossified, have coalesced distally, enclosing the caudal canal, to 

 form the so-called " chevrons," which are thus proved to be the direct derivatives of 

 primary paired intercentra. 



Stage S. Viewed in median longitudinal section, the salient advances at this period 

 are seen to be four in number. Firstly, the ossific tracts, described in the previous 

 stage (PL I. fig. 10, o.v.), when viewed in transverse section (PL II. figs. 4 & 5), are 

 seen to be extending into the lateral cartilaginous expansions, which are being rapidly 

 absorbed over the areas marked f . Internally to these ossific centres and immediately 

 surrounding the notochord, there are marked signs of calcification revealed by the 

 selective action of our reagent. Exactly the same processes take place in the ossifica- 

 tion of the centrum of the atlas (os odontoideum) as of an ordinary vertebra (cf. PL I. 

 fig. 9V). 



The advance in ossification of the vertebral bodies (o.v., PL I. fig. 10) is seen to go 

 on hand in hand with the assumption of a more truly hyaline character by those 

 portions which are still cartilaginous. These (as viewed in longitudinal section) are 

 seen at the same time to undergo a marked change in shape, becoming thinned 

 over the centre of bony deposition and thickened at and towards their extremities 

 i. e., an inverse development of bone and cartilage is recognizable, in proportion as the 

 centre of each developing vertebra is approached. 



In the intervertebral regions (t.a. t PL I. fig. 10) the cells of the skeletogenous sheath, 

 still closely crowded, retain, in the elongation of their nuclei, a character originally 



