May 8, 1879] 



NATURE 



31 



of the cartilage, ossifies, it becomes the " ectosteal layer," 

 and is directly related to the cartilage as its ossifying in- 

 vestment ; this is a true endoskeletal lamina. Ossification 

 of the dermis gives rise to " dermosteal " (exoskeletal) 

 bony plates, such as the scales and scutes of fishes, and 

 the scutes of reptiles and armadilloes. The intermediate 

 fibrous tissue, especially in the region of the head, ossifies 

 to form the splints, investing bones, or "parostoses" 

 (exoskeletal). Lastly, the cartilage itself undergoes an 

 osseous change, either by central, superficial, or sub- 

 central "endostosis" (endoskeletal). 



These species of ossification, like other species, are, 

 however, apt to run into one another. 



Fishes.— li we take a survey of the Vertebrata, begin- 

 ning with the suctorial fishes, viz., the lamprey and hag, 

 we find at first nothing but cartilage forming both the 

 exo- and endoskeleton. The main peculiarities seen in 

 the skeletons of these fishes are the peculiar cartilaginous 

 labials, forming the sucking apparatus, and the basket- 

 work of the " extra-branchials," which embrace the huge 

 multiperforate pharynx. The Selachians (shark, skate, 

 and Chimoera), although retaining much that is low and 

 embryonic in their structure, are in many respects the 

 highest and most reptilian of fishes. Their skeletal 

 growths are uncombined ; in their skin are numerous 

 placoid grains or spines, forming the exoskeleton, while 

 in the endoskeleton the first step towards ossification is 

 seen in the calcification of the superficial cells of the 

 cartilage. The labial system is now secondary, a more 

 perfect mouth having taken the place of the sucking- 

 apparatus. The basket-work of the sucking fishes yields 

 bars to strengthen the mouths of the gill-pouches, the gill 

 system being built upon large endoskeletal branchial 

 arches. Limb-girdles, with their paired fins, appear in 

 these fishes. 



In the Chondrostei (sturgeon and paddle-fish (Plani- 

 rostra), which are lower kinds of Ganoids, the slightly 

 ossified cartilaginous endoskeleton is supplemented by 

 outer bony plates, which, in the extinct forms, were often 

 covered with an enameled layer. In the head, especially, 

 these plates are conformed to the underlying parts, 

 although they do not combine with them histologically. 

 In this region they have also a constant tendency to a 

 peculiar alternation of paired and unpaired elements. 

 The scutes of the trunk, although suggesting the segments 

 within, do not actually correspond with them. 



Here we must look for the exemplars of our own 

 investing-bones {parosfoses) which are as yet, however, 

 very generalized. The chondroskeleton now gets true 

 ectosteal plates and sheaths, as well as parostoses. 



But in those Ganoids that are called Holostei, the endo- 

 skeleton rivals that of the ordinary fishes in hardness, and 

 yet the exoskeleton arrives at its highest pitch of perfec- 

 tion. In the head, the dermoskeleton is brought into 

 adaptation to the more important architecture of the 

 inner parts. The most perfect dermoskeleton is seen in 

 the gar-pike (Lepidosteus). 



The Dipnoi, which are in many respects related to the 

 generalized chimosroids, show even less mutual adapta- 

 tion of the outer to the inner skeleton than the sturgeon 

 and paddle-fish, and, moreover, their bony skull plates 

 are, as a rule, feeble, and few in number. 



Lepidosiren and Protopterus have a few subcutaneous 

 bones (parostoses) applied to a cranium which is almost 

 entirely devoid of intrinsic ossifications, and scarcely 

 advanced in development beyond that of a Chimoera. 

 But Ceratodus has a helmeted head much like that of the 

 lower Ganoids, the dermal scutes overlying the almost un- 

 ossified cranium ; it has also some sub-cutaneous bones. 



The osseous fishes are the highest as fishes, but they 

 are least of all related to those types which rise above 

 them in the scale. Their metamorphosis is very great, 

 but the elements are still uncombined. They have a 

 copious growth of sub-cutaneou3 bones, as the Selachians 



have of sub-cutaneous cartilages, while in Ceratodus both 

 are seen in an uncombined condition. 



Amphibia. — The Amphibia are a subdirision of the 

 Icthyopsida, which, like the Dipnoi, develop lungs as well 

 as gills, but which often shed the latter, breathing only 

 by the former. Their embryos, like those of fishes, 

 develop neither amnion nor allantois. There are four 

 orders in this sub-class, viz., the Coecilians, the Urodeles, 

 the Anura, and the Labyrinthodonts, the last of these 

 being the large extinct Amphibia of the coal-measures. 

 The living forms of Amphibia begin life as a sort of 

 fishes, having gills ; and, as a rule, they live in the water 

 until they acquire lungs : some keep their gills, and con- 

 tinue to live in the water, while others shed them. The 

 higher kinds undergo so much morphological change, 

 and assume so many new and important characters, that 

 they are perhaps the most instructive of all the Vertebrata. 



In the Urodeles and Anura (tail-bearing and tail-less 

 Amphibia) we find many things in common, and many 

 more that are different. While they agree in possessing 

 gills in their larval state, they differ in the character of 

 their gills. The Urodeles have, some for a while, and 

 . others throughout life, three pairs of pinnate external gills, 

 attached to the three first branchial arches, a single gill 

 to each arch, there being generally a fourth arch which 

 does not bear a gill. These are true inner branchial 

 arches. 



Amongst the frogs and toads there are three important 

 modifications of the branchial system. In the common 

 kinds (Opisthoglcssd) there are at first free tufted gills 

 growing from the two first, at least, of the four bran- 

 chial arches, all of which are functional. These are 

 soon hidden by an opercular outgrowth from the hyoid 

 arch, which covers over and closes up all the branchial 

 region, leaving, however, a small aperture on the left side. 

 The primary pharyngeal wall not only sphts so as to 

 form four clefts on each side, but the wall itself becomes 

 divided so as to form a series of pouches, each of which 

 has a cartilage within and a cartilage without, the oper- 

 cular skin loosely covering the pouches outside. The 

 intra-branchial arches are small bars ; the second and 

 third extra-branchials are large bars, while the first and 

 fourth are large pouches. Tufted vascular (lophobran- 

 chiaie) growths, like those first seen outside, grow on the 

 inside of the large pouches and bars, and also from the 

 three branchial clefts, outside the extra-branchials. These 

 latter correspond with the external gills seen in unhatched 

 sharks and skate. 



Dactylethra, one of the two existing Aglossa, shows no 

 trace of external gills. The other, tongueless kind, Pipa, 

 has probably very temporary rudiments of them. Suctorial 

 cartilages have nearly disappeared in the embryos of 

 Urodeles, but in the Batrachia they are nearly as much 

 developed as in the lamprey. In the Urodeles we find no 

 trace of a gill on the first and second arches behind the 

 mouth, nor on the sixth, which exists in the majority of 

 the species. 



It is evident that the tailed Amphibia have been 

 dropping from time to time parts no longer useful to 

 them, whilst straining after a higher organisation. In 

 them we have the beginning of the middle ear; there is a 

 stapes and a fenestra ovalis. Here also a larynx appears 

 for the first time, and the shoulder and hip girdles, and 

 the fore and hind limbs are developed similarly to those 

 of the higher types. 



In neither Anura nor Urodeles is it possible to make a 

 sharp distinction between a parostosis and an ectostosis, 

 especially in the palate. The frogs and toads vary greatly 

 in the intensity of their ossification ; the parostoses pass 

 into superficial dermal plates, and the bones, both super- 

 ficial and deep, are apt to begin in a wild way, not 

 keeping to the habitual landmarks. This is seen also to 

 a less extent in Urodeles. 



The mind of man is not able to invent a more wondrous 



