382 MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 
them, there are, in certain species, imperforate processes projecting in the middle line, 
and not even forking to enclose the great vessels." | 
Professor Huxley, speaking of the chevron bones of the Amphibia, says, in his last 
course of Hunterian lectures *, “These processes certainly, however, do not answer to 
ribs, as they coexist with transverse processes to the extremities of which moveable ribs 
are articulated." This is well seen in Menobranchus. 
Now, reverting to the condition of the embryo, and the bifurcation of the ventral 
plates into the external and internal laminee, I think it is most probable that median 
azygos processes beneath the centra of the trunk vertebree belong to the common root 
of the internal laminæ. Such processes are, e. g., those in the lumbar and last dorsal 
vertebre of the Hare, the last cervical vertebre of the Great Auk and Penguin (Plate 
LIII. fig. 11, Hy.), the trunk vertebræ of the Rattle-snake or Cobra, &c. &c. 
Often, e. y. in the Penguin, median azygos.and quite single processes are in series 
with others which begin to bifurcate, and these again with others in which the bifurca- 
ting limbs reunite inferiorly so as to form a subcentral bony ring. 
Such parts in the trunk evidently, if they do not actually belong to the internal 
laminæ, are clearly related to them, and belong to that part of the undifferentiated mass 
from which those laminx elsewhere proceed; they are therefore what I propose to call 
hypaxial parts. : : 
Now as, in the series of hypaxial parts forming the hyo-branchial apparatus and jaws, 
the branchial arches (if any) in front of the bifurcation of the ventral plates into exter- 
nal and internal laminæ are most probably similar in nature to those behind (and 
therefore within) such bifurcation, so the processes, ridges, or arches in parts of the 
body where there may be no distinct differentiation into external and internal laminæ 
must be held similar in nature to the serially homologous ridges, processes, and arches 
developed in parts of the body where there is such a differentiation. 
Again, just as the aortic arches may be taken as indicating the potential line of such. 
differentiation beyond its actual limit, so the continuation backwards of the dorsal aorta 
may similarly be taken as a potential (if not actual) division—the parts immediately em- 
bracing it in the tail being evidently serially homologous with the parts embracing it in : 
the trunk. Now the dorsal aorta in the trunk belongs surely to the system of the internal 
laminæ and their common root, the external lamine lying altogether without and sur- 
rounding the visceral cavity. "Therefore any hard parts immediately embracing the 
caudal continuation of the dorsal aorta must belong to the same system (i. e. to the 
internal laminz), unless, by the atrophy of the visceral cavity, parts serially homologous 
with ribs are brought into immediate contact with the eaudal vessel, and coalesce or are 
connate with or replace and simulate absent hypaxial parts. 
The coexistence, however, in the tail, of the transverse processes of both kinds 
(tubercular and capitular) with chevron bones, as in Menobranchns, proves that the 
latter bones cannot be the homologues of the paraxial hard parts of the trunk (as has 
already been said); while the fact of their being embraced externally, in the Crocodile, 
by the lining membrane of the continued visceral cavity (as observed by Professor 
iin * Loc. cit. p. 279. | 
