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Show 90 ON CLASSIFICATION. Ina1 e s are 1o C1L ge c1 in ca. sr.rotum ' w• hich is s• nsp nd d in front of t h o peni·s ; an d the vasa d ferent1a op n Into a co. mplete a.n d con tI.l lUOUS UI. thi''"u' ·, which is also the pn ·ago. b:y wlnch the m·mo escapes f rom the, bladder' and is per} tly di t1uct. from. tho pas- I! tl A sage 10r · 1v f:<:nces thouo·h the anus and tho t nninatwn of tho c.o , b . urethro-sexual canal are en1Lraced b~ the smne spln:H:t er. . The corpus callosum is con1parativ ly stnall, as In tho 01'ntt-thodelphia. . . . . It is stated that the allantm of the einLryo 1s arr st d 111 Its development, and gives ri. e to ~o plac. nta. The umLil~cal sac is said to acquire a largo proportwnal 1~ ; bu~ whether .Jt pl~:ys the part of a placenta for th . hort p nod of 1ntra-uten uo hfo, or not is unknown. The young are born f ~ ry small. siz , and in a singnl:nly imperfect condition; Lut b 1ng tr~lJSferr d to t1Jo mat:suplll.m, . and becoming attached to a long nipple, th y are supphed w~th milk until they are able to provide for th In ·~v -tho nu]k being, at first, forced into th ir mouths by the action of a musdo spread over the n1an1mary gland. . . In the 1\IoNODELPIIIA, the angl of the lower Jaw 1s not inflected and they may or Inay not bo provided with teeth. They ne~er possess "marsupial" bone . The uterine dilatation of the oviduct is alway consid rabl , a.ncl wh ther th y have common or distinct a1 ertures, th vagina is a single tube, though it may be partiaUy divided by a ptum. :rho testes may vary much in position; but, if they are lod~ed 111 a scrotal pouch, it is never p nclulon by a narrow neck Jn front of tho penis, as in the Didelphia. . The urinary bladuer opens into a di tin t urethra, whwb, directly or indirectly, r ceives the vasa c1 ferentia in the male. The corpus callosum is very variaLle in its developm~nt, con1monly attaining a mu ·h larger size than in the prccodmg groups; the optic lobes are divid d into four portion.s. . The young are nouri h d within the ut rus until such tune as they are competent to suck milk from the teats of tho par~n~, to which end the chorion always develop processes or vJlh, which are well supplied with vessels Lrought to them by the allan toil:). Those proc ., . b co min o· int rlaced more or loss THl(j PLACENTALIA AND IMPLACENTALIA. Dl closely with corresponding vascular d velopment of the wall of the uterus (and so forming a "placenta"), an interchange of constituents takes place between the frotal and the Jnaternal blood, through the separating walls of tho frotal and maternal vessels. In this manner, throughout its prolonged intra-uterine life, the Monodelphian footus is supplied with nourishment and gets rid of its effete products. As the three groups instituted by De B1ainville are capable of Leing thus clearly differentiated one from the other, the distinctions between them having been only more and more clearly brought out by the subsequent progress of knowledge, I can see no gronnd for refusing to adopt his classification, or for denying him that credit to which he is fairly entitled for apprehending these distinctions. Certainly, the later proposition, to divide Matnmals into two great groups only, Placentalia and Implacentalia, cannot be regarded as any improvement upon De Blai~ville's systen1, as it. ignores the important fact that the Fig. 39. Fig. 39.-Diagmmmatic section of a human preO'nant uterus with the contained 0\'Um \\on~et). 1t, ~1terus; l, oviduct; o, cervix"' uteri; du, decidua uteri; dr, decidua IC l~xct; ds, decidua scrotina; ch, chorion,· am, amnion· al allantois· nb umbilical VCSICie ,' .z, v 1'I I" 1 . 1. {' 1 ' ' 1 ' '. w ucn orm t 1C footal part of the placenta; z', villi over the rest of the chorJOu, wluch take no part iu the placental function in man. |