334 report — 1846. 



ments, whether retaining their spinal shape as in the caudal haemapophyses, 

 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 haemal 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 haemal 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 (hagmal) peripheral elements 

 of the vertebrae ; 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 ha?mal 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- 

 bras. In the composition of the neural arch of the occipital, parietal and 

 frontal vertebra?, we find the neurapophyses repeating the pleurapophyses of 

 the haemal arch, and the parapophyses repeating the haemapophyses 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, I 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 A natomy, 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, pi. 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 hones of the fore-arm and leg, as follows : — " On com- 

 paring the atlantal (pectoral) and sacral (pelvic) extremities, the fibula is found to he the bone 

 corresponding to the ulna ; and accordingly, upou extending our researches to Comparative 

 Auatomy, 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. 



