STRUCTURE AND DEVELOPMENT OP LEPIDOSTEUS. 393 
To this subject we shall return again at the end of the section. 
Before leaving the haemal arches it may he mentioned that behind the region of the 
ventral caudal fin the two haemal processes merge into one, and form an unpaired knob 
resting on the ventral side of the notochord, and not perforated by a canal. 
There are now present well-developed intervertebral rings of cartilage, each of which 
eventually becomes divided into two parts, and converted into the adjacent faces of the 
contiguous vertebrae. These rings are united with the neural and haemal arches of the 
vertebrae in front and behind. 
Each ring, as shown by the transverse section (Plate 28, fig. 71), is not uniformly 
thick, but exhibits four projections, two dorsal and two ventral. These four pro¬ 
jections are continuous with the bases of the neural and haemal arches of the adjacent 
vertebrae, and afford presumptive evidence of the derivation of the intervertebral rings 
from the neural and haemal arches; in that had they so originated, it would be natural 
to anticipate the presence of four thickenings indicating the four points from which the 
cartilage had spread, while if the rings had originated independently, it would not be 
easy to give any explanation of the presence of such thickenings. Gegenbaijr (No. 6), 
from the investigation of a much older larva than that we are now describing, also 
arrived at the conclusion that the intervertebral cartilages were derived from the neural 
and haemal arches; but as doubts have been thrown upon this conclusion by Gotte, and 
as it obviously required further confirmation, we have considered it important to 
attempt to settle this point. From the description given above, it is clear that we 
have not, however, been able absolutely to trace the origin of this cartilage, but at 
the same time we think that we have adduced weighty evidence in corroboration of 
Gegenbaur’s view. 
As shown in longitudinal section (Plate 28, fig. 69, iv.r.), the intervertebral rings 
are thicker in the middle than at the two ends. In this thickened middle part the 
division of the cartilage into two parts to form the ends of two contiguous vertebrae 
is subsequently effected. The curved line which this segmentation will follow is, how¬ 
ever, already marked out, and from surface views it might be supposed that this 
division had actually occurred. 
The histological structure of the intervertebral cartilage is very distinct from that of 
the cartilage of the bases of the arches, the nuclei being much more closely packed. 
In parts, indeed, the intervertebral cartilage has almost the character of fibro-cartilage. 
On each side of the line of division separating two vertebrae it is invested by a 
superficial osseous deposit. 
The next oldest larva we have had was 11 centims. in length. The filamentous 
dorsal lobe of the caudal fin still projected far beyond the permanent caudal fin 
(Plate 21, fig. 16). 
The vertebral column was considerably less advanced in development than that dis¬ 
sected by Gegenbatjr, though it shows a great advance on the previous stage. Its 
features are illustrated by two transverse sections, one through the median plane of a 
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