432 MINOT. [Vot. II. 
When the ovum reaches the uterus, it appears to exert a 
more direct influence, for one set of changes occurs in the 
placental area, where there is concrescence of foetal and mater- 
nal parts; another in the region around the placenta (peri-pla- 
centa, decidua reflexa), and still another in the rest of the uterus 
(decidua vera, ob-placenta). Whether the three zones enumer- 
ated can be distinguished in the pregnant uteri of all placental 
mammals, and whether they have more features in common than 
appear from a direct comparison between man and the rabbit, 
are questions to be decided by increased knowledge. However, 
it already seems very probable that the decidua reflexa and 
peri-placenta are homologous at least in rodents. 
Concerning the evolution of the amnion nothing definite is 
known, nor do the speculations of Balfour (Comparative Em- 
bryology, I1., 256) nor of van Beneden and Julin, 44, 425, seem 
satisfactory, although the view of the latter is suggestive. 
They say :— 
«Tans notre opinion, la cause determinante de la formation de 
Venvellope amniotique réside dans la descente de l’embryon, déter- 
minée elle méme par le pois du corps. C’est par une accélération du 
développement que la cavité amniotique en est venu a se former quand 
lembryon ne possede encore qu’un pois insignifiant.” The chief ob- 
jection to this theory is that it really gives no cause for the expansion of 
the somatopleure and chorion; there is no proof that a mere strain of 
weight can cause the cells of a membrane to proliferate, and since such 
proliferation is the immediate cause of the growth of the amnion, van 
Beneden and Julin must assume for their theory that the strain of weight 
does cause proliferation ; but this assumption lacks support. Moreover, 
they give no evidence to show that the embryo zz w#ero is situated in 
the primitive amniota upon the upper side of the ovum, although it is 
probable such is the case.”’? 
Ryder’s theory, 19, of the origin of the amnion, like that of 
van Beneden and Julin, to which he does not refer, is purely 
mechanical ; but Ryder seeks the cause in a rigid zona radiata, 
forcing the embryo down into the yolk. See his summary, /c., 
p. 184. So far as we know, however, the embryo of the Sau- 
ropsida cannot be said to sink into the yolk, and so lead to the 
development of an amnion ; but, on the contrary, the amniotic 
1 Quoted from Buck’s Reference Handbook, I., 140. 
