OCR Text |
Show 92 ON CIJASSIFICATION. d. · · of the I 1nplacentalia ar eparatecl by characters two !VISIOnS b l . l d' . . of fully as great iro.portan~e as t osc w nc 1 1stmgn1 h the Placentalia and Implacentaha. . . . But whether the Ornithodelph~a and th D~delph~a are re-garded (as I believe they oug:~t to bo) as two of tho .three pn·m ary separ·ate "sub-clas s of the lass Marnmaha, or G • . • • • whether they are looked upon only a. suodlVJSIOllS of tho Im-placentalia, there is no doubt that th:~ ~re, and w~ll re1naiu, dis- . tinct natural assero.bla.g , the subd1v1 1 ns of whiCh present no very great difficulti s. . . It is otherwise with the ub-class 1lfonodeph2a-·wlnch con-tains at least a dozen order , the arrange1nent of which into groups, not only in detail, but in principle, i , and long has been, a subject of much diffi<'ulty, and on cqu ntly of controv~r~y. Sir Everard Homo* i om1nonly quoted as the or1gmator of one of the two ystems of la ifi ation in Yoguo at tho pres nt day; but his vague tat m nt. and ronfuseJ notions r specting the varyina chara ter of th placenta of tho Monode~Jhia hardly ntitle him to that honour, which, in my opinion, belongs rather to that min nt 1uan, l(arl Ernst von Baer, of whom it can be truly aid that h ha ton ·hod no subject without throwing a flood of light upon it. Toward the end of his famous s ay, ' U nt r u hung n tiber die Gofasverbindung zwisch n l\Iutt r und Fru ht," published in 1828, the following passage occur :- " In the first plac> , I h( e taken pain · to how that tho ova of mammals are nly varit tion of (. singl type ; and if we except the oya of the Mar upial , · n erning which I cnn form no judgment, all on i t of th an1 I art ; all have a placenta ; and, in all, m porti n f th chorion is smooth. The footal placenta c n i t v rywh r of the arne elomen~s, but offers the mo t r markable differ nc in its external dis-position. It is either- 1. Merely appli d to th mat rnal plac ·nta, and (a) continuous and zon -lik . First form. (b) divided into 1nany part . 1econcl form. * "Comparaiiv Auaiumy," vol. iii. VON BAER AND ESCIIRICHT. 93 Or 2. It and the maternal placenta grow together, and they lie, (a) in a zone round the egg. Third form. (b) at one end of it. Fourth form. These differences, however, are developed gradually, and, at first, are less marked." The first form, described in the text of the work, is that met with in the pig. It is what is now co1nmonly termed a diffused placenta ; but Von Baer, more accurate than mo t of his successors, indicates the confinement of the placental villi to the middle of the chorion-its prolonged poles remaining bare-by the term "giirtelformig," zone-like. The second form is that exemplified by the cow and sheep, the cotyledonary placenta. The third is the carnivorous placenta, termed zonular. The fourth is the placenta of man, called now-a-days discoidal. The most important circumstance pointed out by Von Baer, however, is one which has been greatly overlooked, if not wholly ignored, in subsequent discussions-the fact that th ~ differences in the for·m of mammalian placentre are .subsidiary if compared with their differences in structure, more particularly in regard to the extent to which a maternal element enters into their composition. Eschricht, in the ad1nirable mmnoir, "De Organis qum Respirationi et N utritioni Footus Mammalium inserviunt," whiuh he published in 1837, repeats the idects of Von Baer, apparently without being aware of the fact, and enlarges upon them as follows (p. 30) :- ,, Restat, ut succinctam expositionern Mammalium afferamus secundum varias qure in iis observantur, placentre formas. ''A ceteris omnibus mam.malibus Marsupialia et Monotremata separanda sunt, quibus nulla est placenta. Cretera omnia in duas familias dividenda, quarum alteri placenta uterina caduca, alteri non caduca est. Huic Mammalia primata t ungulata omnia adnumeranda sunt, inter qure Ruminantia ob singularem coty ledorum formam creteris opponi possun t. "In mmnmalibus placentmn uterinam caducam habentibus |