296 PROFESSOR ROLLESTON ON THE 



early stage of impregnation." Preparation no. .3466 is, beyond doubt, a preparation of 

 an organ in much the same condition as the organ figured by C. G. Carus in the plate 

 just referred to. But I submit that the words which follow those which I have just 

 quoted from the Hunterian Catalogue, viz., " each of the embryos is contained in a 

 special dilatation appended to the side of the uterine tube," do not apply to the struc- 

 tures in that preparation. For, firstly, what is spoken of as a " special dilatation 

 appended to the side of the uterine tube " is in reality a conical projection with its wall 

 curving continuously with those of the uterine tube, and not forming, as in my figure 8, 

 segments of another circle. And, what is of more consequence, the embryos are contained 

 in the uterine tube, and what is contained in the misnamed " special dilatation appended " 

 to it is merely serotinse and placenta. The embryo of the Rat, when only a line in 

 length, is contained in the uterine tube, and, together with its envelopes, causes the 

 calibre of that tube to bulge outwards on both sides between its mesometrial and its 

 free border; and a reference to Reichert's plate 6, in his recently published and most 

 valuable essay, ' Beitrage zur Entwickelungsgeschichte des Meerschweinchens,' will 

 satisfy every one who may not have access to yet earlier specimens, that, in all proba- 

 bility, it never occupies any other position'. In a word, this developing uterus in the Rat 

 is moniliform, and is correctly so represented, for the most part, in Carus's plate ; it is 

 the uterus retrocedent post partum, which has " special dilatations appended " to it, and 

 thus presents in section a figure somewhat resembling a figure of 8 as ordinarily written, 

 with its upper segment the larger of the two. 



The foetal membranes of the Rat at earlier stages are especially instructive, and 

 without a study of them the homologies of the adult envelopes are scarcely to be un- 

 ravelled. When the foetus is five eighths of an inch long, there are two structures in 

 connexion with it, the relations, and proportions, and even the existence of which are 

 much masked in the more advanced stages of its development. These structures are, 

 firstly, the decidua reflexa, which forms at this period a perfect capsule for the foetus, 

 but which near full time is usually represented by the thickened rim at the periphery 

 of the deciduous serotina alone, though a few shreds may still remain, and in the 

 Water-rat (Arvicola amphibia) often do remain, appended to this thickened rim, as if to 

 indicate its real import. Kolliker's words (I. c. p. 154), " Gibt es eine Stelle wo man den 

 Uebergangvon Zellen in Bindgewebsfasern deutlich demonstriren kann, so ist es hier," 

 apply most accurately to this thickened rim, the remnant of decidua reflexa. The 

 second structure, seen jDlainly in the early, and obscured or lost in the more advanced 

 Rat-embryo, is the primitive chorion. This membrane will be seen to pass from the 

 point of attachment of the secondary chorion to the centre of the placenta outwards, 

 to line the internal surface of the capsule of decidua reflexa. It is somewhat strange 



Vou Baer's remarks upon this subject are much to the point — "Alle Embryonen (mit Ausnahmen) der 

 friihesten Zeit normal so liegen class ihr Riickeu in der grossen Curvatur des Fruchthalters und seiner Horner 

 licgt." Entwickelungsgeschichte, ii. p. 232. See also Reichert, I. c. pp. 130 & 131. 



