594 CLASS XVII. 



is an independent act of tlie female genital organs unconnected 

 with copulation. The ovum is now received by the mouth of the 

 Fallopian tubes and conveyed into the cavity of the uterus. The 

 time required for this transit is very different in different mammals, 

 from three to ten or twelve days or even more. The membrane 

 which encloses the ovum [zona pellucida) during the passage through 

 the tube is fortified by a layer of albumen, and at a later period is, 

 in connexion with the peripheric part of the animal layer of the 

 germinal vesicle, developed into the chorion by which the embryo in 

 the uterus is invested as an external covering. Further, this chorion 

 in man and the apes is surrounded on the outside by investments 

 which do not properly belong to the ovum, but are products of the 

 uterus {inemhrana decidua vera and reflexa). By these or by the epi- 

 thelium of the mucous membrane immediately the ovum is brought 

 in connexion with the uterus. Afterwards a closer union is effected 

 by means of the blood-vessels which with the allantois penetrate 

 to the chorion; in this way the placenta is formed, a rooting or 

 implanting of the ovum in the uterus by means of the umbilical 

 vessels ; it occurs in the mammals alone, but is however wanting 

 in the Marsupiates and Monotremes, in which the allantois conti- 

 nues small and does not extend as far as the chorion. When the 

 allantois grows round the entire ovum, the 'placenta may extend 

 over the whole chorion (pachyderms, solidungulates), or may be 

 collected into small groups as eminences on the chorion (ruminants), 

 or form a ring round the ovum (carnivores). In the rodents, the 

 apes and man the allantois is connected with a limited part only of 

 the chorion, and to this part alone the formation of the placenta is 

 restricted. But notwithstanding this connexion, neither in man, 

 nor in the mammals is there any immediate passage of the blood 

 from the vessels of the mother into those of the foetus, nor, con- 

 versely, from those of the foetus into those of the parent animaP. 



^ The development of the embryo in mammals proceeds in a mode similar to that in 

 birds (see above, pp. 352 — 357). In addition to the second part of VoN Baek, Ueber 

 EntunchelungsjeschicJUe der Thiere, Konigsberg, 1837, s. 164 — 279, four excellent works 

 of Th. L. W. Bischofp may here be studied : Entwiclelungsgeschichte des Kaninclien- 

 Eies, Braunschweig, 1842, 4to, Enticiclcelimgsgesch, des Himde-Eies, ibid. 1845, 4to, 

 EnhoickeiungsgexnJi. des MeerscMoeinchens, Giessen, 1852, 4to, and Eniiclchelungsgesch. 

 des Relies, ibid. 1854, 4to. (Here, in Cavia cohaya, the singular anomaly occurs, that 

 the mucous layer lies on the outside of the germinal membrane, and that thus the 

 embryo has its back turned to the inside of the germinal vesicle.) Amongst the 



