AND THOSE OF CERTAIN OTHER MAMMALIA. 89 



metrial and its free border ; and a reference to Reichert's plate 6, 

 in his recently published and most valuable essay ' Beitrage zur 

 Entwickelungsgeschichte des Meersehweinchens,' will satisfy every 

 one who may not have access to yet earlier specimens, that in 

 all probability it never occupies any other position 1 . 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 unravelled. 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 deve- 

 lopment. 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 (1. 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 that Bischoff, who has 

 figured what is nothing else than this primitive chorion (fig. 

 59, Meerschweinchen-Ei), should have yet fallen into the error 



1 Von Baer's remarks upon this subject are much to the point : ' Alle Embryonen 

 (mit Ausnahmen) derfruhesten Zeit so liegen dass ihr Riickenin der grossen Curvatur 

 des Fruchthalters und seiner Horner liegt.' Entwickelungsgeschichte, ii. p. 232. 

 See also Reichert, I.e. pp. 130 and 131. 



