164 
Farre^ on an Em^ly Human Embryo. 
by reference to any of the ordinary modes of calculation. 
In this case two menstrual periods liad passed by before the 
occurrence of abortion. But it is plain_, from the condition 
of the embryo^ that any calculation^ having reference to this 
circumstance^ would make its age far greater than the small 
amount of development which it has undergone would war- 
rant. I have therefore no alternative but to estimate the 
period of development in this^ as in the former instance^ by 
comparing the embryo with others, the age of which could 
be more accurately ascertained. Judged by this test_, I am 
inclined to fix the period of development of the present 
example at the early part of the fourth week of utero- 
gestation, and_, as compared with the specimen which I for- 
merly desciibed_, as probably three or four days younger 
* than it. 
The external characters of the ovum in this case were such 
as are commonly observed in the human ovum when expelled 
by abortion. It was loosely surrounded by the decidua vera 
and reflexa. The ovum having been freed from these, was 
found to be of nearly spherical form, the chorion being 
everywhere covered by dendritic villi, which were nearly 
equally distributed over its surface. The extremities of these 
villi were only slightly attached to the decidua, except in one 
situation where the two surfaces were united for a small 
extent. Taking the chorion as the boundary of the ovum 
proper, it measured one inch in diameter. 
Upon carefully laying open the chorion with a pair of 
scissors under water, the embryo, the seat of which could be 
already discerned through the semi-opaque membrane, was 
at once brought into view. It was not enclosed in a separate 
amniotic sac, but the amnion appeared to be everywhere 
adherent to the inner surface of the chorion by the interven- 
tion of a loose connective tissue (magma reticule), so that 
the amnion, in this instance, was either not yet fully deve- 
loped, or had not its usual distinct form. 
The embryo itself, to which T will now confine my descrip- 
tion, was attached to the parietes of this inner sac by a short 
peduncle springing from the posterior extremity of the trunk, 
just between the rudimentary buds which indicate the seat of 
the future lower limbs. This was, I presume, the root of the 
allantois, and in opening the semi-opaque sac through which 
I could see the situation of the head of the embryo, and in 
keeping as far as possible from this point, so as to avoid 
mutilating what I knew would be a very interesting part, I 
unfortunately cut across the pedicle of attachment, and so 
by trying to avoid Scylla I fell into Charybdis. I men- 
