STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
395 
A comparison of the neural arch at this stage with the arch in the adult, and in the 
stage last described, shows that the greater part of the neural arch of the adult is 
formed of membrane-bone, there being preformed in cartilage only a small basal 
part, a dorsal process, and paired key-stones below the ligamentum longitudinale 
superius. 
The haemal arches (Plate 29, fig. 78) are still largely cartilaginous, and rest upon 
the sheath of the notochord. They are invested by a thick layer of bone. The bony 
layer investing the neural and haemal arches is prolonged to form a continuous 
investment round the vertebral portions of the notochord (Plate 29, fig. 78). This 
investment is at the sides prolonged outwards into irregular processes (Plate 29, fig. 78), 
which form the commencement of the outer part of the thick but cellular osseous 
cylinder forming the middle part of the vertebral body. 
The intervertebral cartilages are much larger than in the earlier stage (Plate 29, 
figs. 77 and 79), and it is by their growth that the intervertebral constrictions of the 
notochord are produced. They have ceased to be continuous with the cartilage of the 
arches, the intervening portion of the vertebral body between the two being only 
formed of bone. They are not yet divided into two masses to form the contiguous 
ends of adjacent vertebrae. 
Externally, the part of each cartilage which will form the hinder end of a vertebral 
body is covered by a tube of bone, having the form of a truncated funnel, shown in 
longitudinal section in Plate 29, fig. 77, and in transverse section in Plate 29, fig. 79. 
At each end, the intervertebral cartilages are becoming penetrated and replaced by 
beautiful branched processes from the homogeneous bone which was first of all formed 
in the perichondrium (Plate 29, fig. 77). 
This constitutes the latest stage which we have had, 
Gegenbaur (No.- 6) has described the vertebral column in a somewhat older larva 
of 18 centims. 
The chief points in which the vertebral column of this larva differed from ours are: 
(1) the disappearance of all trace of the primitive vertebral constriction of the noto¬ 
chord ; (2) the nearly completed constriction of the notochord in the intervertebral 
regions; (3) the complete ossification of the vertebral portions of the bodies of the 
vertebrae, the terminal so-called intervertebral portions alone remaining cartilaginous: 
(4) the complete ossification of the basal portions of the haemal and neural processes 
included within the bodies of the vertebrae, so that in the case of the neural arch all 
trace of the fact that the greater part was originally not formed in cartilage had 
become lost. The cartilage of the dorsal spinous processes was, however, still 
persistent. 
The only points which remain obscure in the later history of the vertebral column 
are the history of the notochord and of its sheath. We do not know how far these 
are either simply absorbed or partially or wholly ossified. 
Gotte in his memoir on the formation of the vertebral bodies of the Teleostei 
