Lectures on the Elements of Comparative Anatomy

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88 ON CLASSIFICATI?N · ledge but I s1 1 a ll du. ect Y0 UI' attention at once to th·a t importa. nt step t' owar d s a· ·a· the Mamrnalia into large groups, wh1c·h IVI 1ng . . . was tak en by the emi· nent French anato.n ns. t, M. .d e Blainville, so f:a r ba c k as th e ye al. 181G · M. de. BlainvJlle po.i nted out that h t e 171utra mma [t't a mi• gh t be divided Into three .p rnnary groups, accor dI. ng t o tl1 e ch aracter of their reproductive o.r g.a ns, espe- C·i a 11 y tl1 e repro duct1've organs"' of the female. He d1v,1 ded them into " Ornithodelphes," "Didelphes," "Monodelphes; or, as ·we · ht t . thmn ORNITHODELPHIA, DIDELPHIA, MoNODEL-mig erm ' M d Bl . '1 PHIA. N OW, I do not mean to assert that . e ainvl .l e defined these different groups in a manner altogether sails-factory, or ·strictly in accordanc~ with all the subsequently dis­covered facts of science, but h1s great knowledge and acute intuition led him to perceive that the groups thus named were trul natural divisions of the Mammalia. And the enlargement of :ur knowledge by subsequent investigation seems to 1ne, in the main, only to have confirmed De Blainville's views. The division of the ORNI'l'HODELPHIA comprises those two remarkable genera of l\{ammals, as isolated in geographical distribution as in strncture,-Ornithorhynehus and Echidna,­which constitute the order Monotremata. In these anima1s the angle of the lower jaw is not inflected, and the jaws are devoid of true teeth, one of the two genera only ( Ornithorhynehus) possessing horny plates in the place of teeth. The coracoid bone extends from the scapula to the sternum, with which it is articulated, as in birds and most reptiles, and, as in many of the latter, there is an episternal bone. Ther~ i'~ no marsupial pouch, though bones wrongly termed "marsupial are connected with the pelvis. But it is to the structure of the female reproductive organs that the Ornithodelphia owe their name. The oviducts, enlarged below into uterine pouches, but opening separately from one another, as in oviparouR vertebrates, debouch, not into a distinct vagina, but into a cloacal chamber, common to the urinary and genital products and to the freces. The testes of the male are abdominal in po ition throughout life, and the vasa deferentia open into the cloaca, and not into a distinct urethral passage. Tho penis i~ ind ed traversed by an THE OHNITHODELPHIA AND DIDELPHTA. 89 urethral canal, but it is open and interrupted at the root of that organ. In both sexes, the ureters pour the renal secretion, not into the bladder, which is connected with the upper extremity of the cloaca, but into the latter cavity itself. In the brain, the corpus callosum is inconspicuous, though the question how far it can properly be said to be absent requires much more thorough investigation than it has yet received.* vVe are but very imperfectly acquainted with the reproductive processes of these animals, but it is asserted that the young are devoid of a placenta. The mammary gland has no nipple. Like the Ornithodelphia, the eli vision DIDELPHIA contains but a single order, the Marsupialia, the great majority of which, like the Ornithodelphia, inhabit Australia. They almost all have the angle of the lower jaw inflected, and all possess true teeth. The coracoid is, as in the higher Mammals, anch y losed with the scapula, and is not articulated with the sternum. All have the so-called " marsupial " bones or cartilages-ossifica­tions, or chondrifications, of the internal tendon of the external oblique muscle of the abdomen-and the females of almost all possess a fold of the skin of the abdomen above the pubis, con­stituting a "marsupium," or pouch, within which the young are nourished and protected in their early, helpless condition. ~rhe oviducts open into vaginre, which are more or less com­pletely divided into two separate passages. rrhe testes of the * For a number of years I have entertained the gravest doubts respecting the accuracy of the doctrine put forth now nearly thirty years ago by Professor Owen, and almost universally received, that the corpus callosum is abf:!ent in Monotremes and Marsupials, and at one time I began to collect materials for the thorough in­vestigation of the question; but other occupations intervened, and the plan was never carried out. Nevertheless, I have always expressed myself cautiously on this subject, and, as the text shows, I was particularly guarded when deliverinO' the present lecture. At that time, in fact, I was well aware that my friend Mr. Fl~wer bad commenced a series of inquiries into the question, and such results as he had then obtained tended greatly to the increase of my scepticism. Mr. Flower has since been g~od eno~gh to go carefully with me over the large series of drawings and preparatiOns whiCh he has made; and I am prepared to express my entire concurrence in his co~clusion that the cor-pus callosum exists, distinctly developed, though not so well as m monodelphous, or placenta], Mammals, in both the Didel­phia and tho Ornithodelphia.