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Show 208 DR. EL GADOW ON THE EVOLUTION [Mar. 18, loosely in the skin, and even shift their position across the suture, and fuse very late with the cranial bones. He naturally combats Riitimeyer's comparison of the antlers with the Giraffe's " horns," which he seems to look upon as sui generis. Not a few writers, amongst them Nitsche and Roeiig, are not clear about the meaning of the somewhat unfortunate terms " Hautknochen," dermal or membrane-bones. Roerig, for instance, thinks that thereby are meant epidermal organs. In reality they are contrasted with cartilage-bones as membrane-bones. To call the latter promiscuously dermal bones has caused endless confusion. A necessary condition for ossification is the presence of an amorphous ground-substance or matrix, which is then converted into, or rather supplanted by, bony tissue. Ossification is consequently always a secondary process. Unless the ground-substance is preformed as amorphous embryonic tissue, it has first to be produced out of existing adult cartilage or other connective tissue by the action of the osteoclasts or similar katabolic, histioclastic cells, which by their breaking-down action upon the tissue dissolve the latter into a medium in which osteoblasts can live, multiply, and by excreting or attracting and arranging around themselves certain salts, turn into bone-corpuscles. On the surface of regenerating bone the marrow-cells, giant-cells, myeloplaxes, seem to produce this ground-substance. In the case of cartilage this is first destroyed, one might as well say dissolved, by the cells which immigrate through the perichondrium, a process which happens frequently when membrane-bone comes into contact with cartilage. It was a great step forwards when it became understood that the place of origin of all bone-forming cells was to be referred to the so-called basal membrane of the epidermis, whence osteoblasts infiltrated or invaded the corium or mesodermal portion of the skin. Recent observations warrant us to go a step further, and to assume that the original home of all skleroblasts was in the Malpighian layer of the epiderm itself. The oldest immigration of skleroblasts from the ectoderm into cutis and other mesodermal tissue has formed cartilage; the next immigration of skleroblasts has given rise to bone. The latter being superior, supersedes the cartilaginous skeleton. The ectoderm has by no means lost the capacity of producing either kind of skleroblasts. Extraordinary excitement and requirements, reactions upon external stimulus, produce this rejuvenescence, even in the mammalian skin. Exquisite examples of true dermal bones are those ossifications " within the skin " which in Amphibia and Reptiles are now generally called osteoderms. They occur also, among Mammalia, in the Armadillos, but in no other group of this class, unless it were in the Cetacea, where Kuekenthal has found traces of a dermal armour. In the Amphibian Ceratophrys ornata the "dorsal shield," although very thick itself, has sunk in so deeply that it is now in contact with the vertebral processes and is covered by the ordinary, movable skin. In Pelobates the skin of the upper surface of the head is partly co-ossified with the underlying cranial bones, giving them a pitted appearance. Now, frontal and parietal being membrane-bones, or at least membranes which have received their bone from the cutis, this superimposed ossifying mass of Pelobates is a second instalment, or second generation of dermal bone. Similar successive repetitions of the same process are demonstrated in those Amphibian and Reptilian vomers which carry teeth, the vomers themselves having resulted from the fusion of the basal portions of teeth which themselves are n ow lost.-Concerning the cranial membrane-bones, there is no doubt that the original cartilaginous roof has vanished (it is restricted to the dura mater), not because its cartilage has been destroyed, or supplanted, by immigrating bony tissue, but because it has been gradually suppressed by the approaching, investing, membrane-bone. Similar instances of suppression, not conversion, are the greater portion of Meckel's cartilage, the premaxilla; and maxillae, probably the palatine and quadrato-jugal bones of Birds and Mammals, and to a great extent the mammalian quadrate through its conversion into the os tympanicum. On the other hand, the human clavicle surrounds, and is intermixed with, the precoracoidal cartilage. The next points of importance considered by Nitsche are the composition and shedding of the horny sheaths of Antilocapra, which he does not homologize completely with the bovine horn-sheaths. He prefers putting the Prongbuck's horns into a position intermediate between the velvet of the Giraffe and the horn of the Bovidae. The following is his terse summary :-The |