STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
391 
which commences as a deposit of bone partly in the perichondrium and partly in the 
intervening membrane, forms a continuous structure. 
Analogous occurrences have been described by Gotte in Teleostei. 
The dorsal portion of each neural arch becomes what we have called the dorsal 
process of the adult arch. 
Between the dorsal processes of the two sides there is placed a median rod of carti¬ 
lage (Plate 28, fig. 70, i.s.), which in its development is wholly independent of the true 
neural arches, and wdiich constitutes the median spinous element of the adult. In 
tracing these backwards it becomes obvious that they are homologous with the inter- 
spinous elements supporting the dorsal fin, in that they are replaced by these inter- 
spinous elements in the region of the dorsal fin, and that the interspinous bones occupy 
the same position as the median spinous processes. This homology was first pointed 
out by Gotte in the case of the Teleostei. 
Immediately beneath this rod is placed the longitudinal ligament (Plate 28, fig. 70, 
l.l .), but there is as yet no trace of a junction between the neural arches of the two 
sides in the space between the longitudinal ligament and the spinal cord. 
The basal parts of the neural arches of the two sides are united dorsally by a thin 
cartilaginous layer resting on the sheath of the notochord, but they are not united 
ventrally with the haemal arches. 
The haemal processes in the trunk are much more prominent than in the preceding 
stage, and their bases are united ventrally by a tolerably thick layer of cartilage. In 
the trunk they are continuous with the so-called ribs of the adult (Plate 28, fig. 70) ; 
but in order to study the nature of these ribs it is necessary to trace the modifications 
undergone by the haemal arches in passing from the tail to the trunk. 
It will be remembered that at an earlier stage the haemal arches in the region of 
the tail-fin were fully formed, and that through the anterior part of the caudal region 
the haemal processes were far advanced in development, and just in front of the caudal 
fin had actually met below the caudal vein. 
The mode of development of the haemal arches in the tail as unjointed cartilaginous 
bars investing the caudal arteries and veins is so similar to that of the caudal haemal 
arches of Elasmobranchii, that it appears to us impossible to doubt their identity in 
the two groups.* 
* Gegenbatjb (No. 6) takes a different view on this subject, as is clear from the following passage in 
this memoir (pp. 869-370) :—“ Each vertebra of Lepiclosteus thns consists of a section of the notochord, 
and of the cartilaginous tissue surrounding its sheath, which gives origin to the upper arches for the 
whole length of the vertebral column, and in the caudal region to that of the lower arches also. The 
latter do not however complete the enclosure of a lower canal , hut this is effected by special independent 
dements , which are to be interpreted as homologues of the ribs.” (The italics are ours.) While we fully 
accept the homology between the ribs and the lower elements of the haemal arches of the tail, the view 
expressed in the italicised section, to the effect that the lower parts of the caudal arches are not true 
haemal arches but are independently formed elements, is entirely opposed to our observations, and has we 
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