372 
(Fig. 4) the anterior neural arches are seen to extend down the sides 
of the notochord, and in the case of the first vertebra the neural 
arches completely conceal the sides of the here already reduced noto- 
chord. The hypochordal cartilage is very broad in this situation, and 
its edges are confluent with the lower ends of the neural arch, so 
that the development of the first vertebra may justly be described as 
perichordal. The vertebral body cannot be distinguished from the 
neural arch, but a diffuse endochondral calcification occurs on the right 
and left sides of the cartilage, and these nearly touch one another 
in the ventral median line. The cartilage is at first continuous with 
that of the skull, but the occipital articulation begins to develop at 
about the same time as the sacral diapophyses begin to enlarge. In 
the stage represented, in Fig. 3 the anterior articular cavities of the 
first vertebra are well formed. The upper part of the neural arch is 
more compact than below, and in texture resembles the neural arches 
of the succeeding vertebrie. 
The neural arches of all the vertebra, including the first, appear 
first as small paired cartilaginous blocks metamerically disposed along 
the dorso-external surfaces of the notochordal sheath. The cartilages 
increase in height and arch inwards, but the roof of the spinal canal 
is for some considerable time composed of a longitudinal band of white 
fibrous tissue. In the specimen shown in Figs. 3 and 4 the right and 
left halves of the neural arches had just met, but were still movable 
upon one another. In a stage only slightly older than this the roof 
was complete and rigid. Ossification takes place long before the roof 
of the neural arch is completed, and, while the lateral portions re- 
main as rounded pillars leaving wide intervertebral foramina, the upper 
portions expand, overlap, and develop paired zygapophyses. The zyg- 
apophysial articulations are very feebly developed between the first 
two vertebr®, so that the neural arch of the first simply overlaps that 
of the second (see Fig. 4). 
In the vertebrae 2 to 9 inclusive the vertebral bodies make their 
appearance as thin, horizontal plates lying flat on the dorsal surface 
of the notochord. They begin to ossify about the same time as the 
neural arches, and have a very coarse-grained texture, in striking con- 
trast with the denseness of the latter. The bone of the neural arch 
is firmly united with the lateral edges of the plate (Fig. 4), but breaks 
away with ease. The plates are thin and approximately square in 
shape, and their outline is sharply marked. The anterior and poste- 
rior edges of adjacent plates are nearly in contact, and the plates are 
slightly movable in a vertical direction at these joints. The centra of 
the second and third vertebree are broader than long and are slightly 
