334 REPORT—1846. 
ments, whether retaining their spinal shape as in the caudal hamapophyses, 
or flattened as ordinary ‘ sternal bones,’ or expanded and subdivided, like the 
neural spines in the cranium, in order to complete below the thorax of the 
bird or to form the plastron of the turtle. 
There reigns a beautiful parallelism in the kind and degree of modification 
of the parts of the neural with the corresponding parts of the hzmal arch of 
the same vertebral segment: and as the serial homologies which have just 
been enunciated succeed each other longitudinally (horizontally in beasts, 
vertically in man) in the axis of the vertebral column, so these manifest them- 
selves in a direction perpendicular to that axis. 
The manubrium sterni of the bat developes a spine downwards, as the 
supraoccipital of the fish sends a spine upwards: the expanded manubrium 
sterni of the whale repeats the condition of the supraoccipital in birds and 
mammals. The form of the ordinary sternal bones in mammals is repeated 
by the parietal and supraoccipital bones of the crocodile. The divided sternum 
of the young ostrich, before the two lateral ossifications have coalesced 
at the median suture, repeats the condition of the divided parietal in most 
mammals. The development of the crista from the obliterated suture of 
the lateral halves of the expanded hemal spine in the thorax of birds is 
paralleled by the development of the crista from the obliterated suture of 
the expanded neural spine in the cranium of carnivores. The interposition 
of the entosternal piece in the chelonian carapace parallels below the inter- 
position of the interparietal bone in the rodent cranium above. 
Thus modifications and developments of the same kind and degree manifest 
themselves in the upper (neural) as in the lower (hemal) peripheral elements 
of the vertebra ; and though not always in the same vertebra, nor in the 
same animal, yet they are sufficiently exemplified in the myelencephalous series 
generally, to establish the conclusion that the hemal spines under all their 
modifications are vertical homotypes, not of the centrums, as Oken, Meckel 
and De Blainville have supposed, but of the neural spines of the same verte- 
bre. In the composition of the neural arch of the occipital, parietal and 
frontal vertebre, we find the neurapophyses repeating the pleurapophyses of 
the hemal arch, and the parapophyses repeating the heemapophyses in their 
relative positions to the centrum and the spine or key-bone of such arches. 
Symmetry or serial homology of parts of the same vertebral segment is 
usually still more strictly preserved in the transverse direction, and is so 
obvious, as to have immediately led to the detection of the homologous parts, 
which are accordingly distinguished as ‘ right’ and ‘ left.’ 
To return to the consideration of those serial homologies with which Vicq 
d’Azyr commenced the study of these relations, | may remark that the bones 
of the fore- and hind-limbs of some of the marsupial quadrupeds best illus- 
trate the true relations which my revered Preceptor in Anatomy, Dr. Barclay* 
was, I believe, the first to enunciate in respect of the bones of the fore-arm 
and leg. 
The skeleton of the Phalangista or Phascolomys plainly demonstrates that 
the tibia is the homotype of the radius, and that the fibula is the homotype 
of the ulna. In both wombat and ornithorhynchus the fibula assumes those 
* In his explanations of Mitchel’s Plates of the Bones, 4to, 1824, pl. 24, figs. 3 and 4, 
Dr. Barclay, without referring to Vicq d’Azyr’s Memoir, simply enunciates the correct 
view of the serial homology of the bones of the fore-arm and leg, as follows :—* On com- 
paring the atlantal (pectoral) and sacral (pelvic) extremities, the fibula is found to be the bone 
corresponding to the ulna; and accordingly, upon extending our researches to Comparative 
Anatomy, we perceive it exhibiting the like variety and unsteadiness of character, sometimes 
large, sometimes small, and sometimes merely a process of the tibia,” &c. He does not push 
his comparison to the bones of the distal segment of the limbs. 
