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 serotinz 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 fcetus, 
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 (J. c. p. 154), ‘‘ Gibt es eine Stelle wo man den 
Uebergang von 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 plainly 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 
Von Baer’s remarks upon this subject are much to the point— Alle Embryonen (mit Ausnahmen) der 
frithesten Zeit normal so liegen dass ihr Riicken in der grossen Curvatur des Fruchthalters und seiner Horner 
liegt.” Entwickelungsgeschichte, ii. p. 232. See also Reichert, /. c. pp. 130 & 131. 
