342 
MR. JONES ON THE IMPREGNATED MAMMIFEROUS OVUM. 
the decidua, rather near its upper edge, and was about the size of a marrowfat pea, 
before being put into spirits. The decidua covering it, towards the hydroperionic 
cavity, was thin and semitransparent, but the opposite portion of the decidual nida- 
mentum was thick, and marked with foramina, as if from vessels which had pene- 
trated and adhered to it. Having opened the nidamentum and taken out the ovum, 
I observed what will immediately strike you, that one side of it was bald and the 
other shaggy with the villi of the chorion. The bald part lay towards the hydro- 
perionic cavity. A small puncture was made through the chorion, and perhaps through 
the amnion, by which some fluid escaped: nothing more was attempted. The Fal- 
lopian portions of the decidua measured nearly half an inch, and were both entire.” 
In a subsequent letter Dr. Mackenzie says, in reference to the age of this ovum, 
“ The ovum in question I consider as three or four weeks old. The lady had missed 
one menstrual period, and thought herself four weeks gone.” 
On laying open the ovum, by carefully cutting and reversing the bald side of the 
chorion, the following appearances (delineated, natural size, in fig. 7-) presented them- 
selves. The whole cavity of the chorion was filled with a fine gelatinous cellular 
tissue, imbedded in which, towards one extremity of the ovum, was a small round body. 
It was evidently the vesicular blastoderma; on being taken out and examined 
under the microscope it presented the same friable globular structure found in the 
vesicular blastoderma of the Rabbit in the preceding observation. There was no 
vitellary membrane to be seen. 
From observation 4. it may be inferred, that in the progress of the development of 
the ovum of the Rabbit the vitellary membrane gives way, as in the ova of the Newt 
and indeed of many of the oviparous animals ; that the gelatinous coat acquired by 
the ovum in the ovary, and more especially circumscribed and defined after impreg- 
nation, constitutes the only covering of the vesicular blastoderma after the giving 
way of the vitellary membrane ; that this gelatinous-looking coat forms the chorion, 
which in the rodents at a further stage of development presents itself under the form 
of a thin and transparent membrane, very like the vitellary membrane of the bird’s 
egg, situated immediately outside the non-vascular and reflected layer of the umbi- 
lical vesicle. 
The conclusions to be drawn regarding the human ovum from observation 5. are 
the same as the above. The human ovum as regards the vesicular blastoderma was 
in much the same stage as the ova of the Rabbit seven days after impregnation ; the 
vitellary membrane had disappeared, or been resolved into the gelatinous cellular 
tissue filling the interior of the chorion ; and the embryo had not yet appeared though 
the vesicular blastoderma was undergoing the preparatory changes. As regards the 
chorion, the human ovum was more developed than that of the Rabbit, but it is to be 
remarked that even in an after stage of development the same difference in structure 
continues to prevail. 
