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Show 70 ON CLASSU'ICA'riON. these appendages, which are term d "feather ," do not ta!\:o tho form of mere plates developed upon the surfa o of tho sk1n, hut are evolved within sacs fro1n the surfaces of conical papillm of the dermis. The external surface of the d rmal papilla, whence a feather is to be developed, is provided upon its dorsal surface with a median groove, which becomes shallower towards tho apex of the papilla. Frmn this median groove lateral funow' proceed at an open angle, and passing round upon tho under surface of tho papilla, become shallower, until, in tho n1iddl lin e, opposite the dorsal median groove, they b co1no obsolete. Minor grooves run at right angles to the lateral furrows. Hence tho surface of the papilla has tho character of n kind of 1nonld, and if it were repeatedly dipped in such a ub tance as a solution of gelatine, and withdrawn to cool until its whole nrfacc was covered with an even coat of that sub tance, it is clear that the gelatinous coat would be thicke t at tho ha. ·al or ant rior nd of them dian groo-ve, at tho median end of the lateral furrows, aHcl at tho ends of the minor grooves which open into th m ;. whil it vYould bo v ry thin at the apices of the 1nedian and lateral grooves, and between the ends of tho minor grooves. If, th erefore, th hollow cone of gelatine, removed from its 1nould, w re ti·otch cl frmn within ; or if its thinnest parts became w uk by drying; it would t end to give way, along the inferior m dian line, oppo it the rod-like cast of the median groove and betw en the ends of the casts of tho lateral furrows, as well as betw en each of tho minor grooves, and the hollow cone would expand into a ilat £ ath r -likc structure with a median s~aft, a a " vane " formed of " barbs " and "barbules." In point of fact, in the deYelop1n nt of a feather such a cast of the dermal papilla is form d, though not in gelatine, but in the horny epidermic layer developed upon tho mould, and, as this is thrust outwards, it op ns out in tho manner just described. After a certain period of growth the papilla of the feather ceases to be grooved, and a continuous horny cylinder is formed, ·which constitutes the "quill." Between Aves and MAMMALIA there is a hiatus, not perhaps, in some respects, quito so wide as that betw en Amphibia and Reptilia, but still very considerable. All Mnm1nals possess an amnion of an et:Jsontially sjmilar THE MAMMAI.JIA. 71 charact r to that of Birds anu Reptiles, and all have an al1antoi . But tho latter either c asos to exist after a very early porjod f frotal life, or else it is "placontiferous," and serves as tho means of intercommunication between the par nt and the off: pring. Of the nature and characters of the "placenta" developed in the mnjority of tho Mam?nalia I shall speak more particularly by and by. For the present, I paRs it over as a structure not universally characteris6c of the class. The visceral arches are, throughout life, as completely devoid of l>ranchjal appendages in Mammals, as in Birds and Reptiles. In the skull, the basi-occipital is well ossified, and, with the oxoccipitals, enters into the formation of tho cranio-spinal articulation; the occipital condyle thus formed, however, is not single, as in Reptiles and Birds, but double, and the atlas has corr·e, ponding articular facets. Fig. 35. Fig. 35.-The occipital condyles of a Dog's skull viewed from bchind.-Signifkation o I' the letters as in Fig. 33. Each ramus of the lower jaw is compos d of only a single piece, a.nd this articulates directly with the squamosal bone of the skull, and not with the representative of the quadrate bone. The greater and lesser circulations of Mammals are as completely di. tinct as in Birds, and there is but a single aortic arch, tho loft. The majority of the blood-corpuscles are r d, froo nu lei, and these are always discoidal, and usually cir ·ular in for1n. The bloocl il:j hot. Thoro is a complete diaphragm, and none of t.he bronehi end in air-sacs. |