208 DR. H. GADOW ON THE EVOLUTION [Mar. 18, 



loosely in the skin, and even shift their position aci'oss the frontO' 

 parietal 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 siti generis. 



Not a few writers, amongst them Nitsclie and Roerig, 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 cavised 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 bj^, bony tissue. Ossi- 

 fication 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 bj- 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, multiplj^, 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 frequentlj' 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 obser- 

 vations warrant us to go a step further, and to assume that the original home of all 

 skleroblasts was in the Malpighian laj^er of the epiderm itself. The oldest immi- 

 gration 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 rejuven- 

 escence, 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 Ceratoplirys 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 Pelohates 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 super- 

 imposed ossifying mass of Pelohates 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 carrj' teeth, the vomers themselves 

 having resulted from the fusion of the basal portions of teeth which themselves are now 

 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 bonj' 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 premaxillas and maxilla3, 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 hoi-ns into a 

 position intei'uiediate between the velvet of the Girafi'e and the 

 horn of the Bovidge. The following is his teise summaiy : — The 



