402 PHILOSOPHY OF ZOOLOGY. 
had no doubt of the other uterus containing a similar em- 
bryo in a less advanced state.” 
The origin of this gelatinous substance has not been sa- 
tisfactorily ascertained. Ais the coats of the uterus are thin, 
it is scarcely to be considered as a secretion from these, but 
is more likely to proceed either from the oviducts or late- 
ral canals. But the manner in which the foetus is nourished 
im this jelly, is a question which remains to be determined. 
The evolution of the ovum by means of placentation, is: 
the most common method observed by Nature, and has 
been longest known and studied by physiologists. The 
ovum is first attached to the walls of the uterus, which are 
lined for this purpose with a layer of lymph, termed mem- 
brana decidua ; a vascular body termed a placenta, is then 
generated, in which the minute branches of the enlarged ute- 
rine arteries terminate, and from which the vessels destined 
to convey the bloed for the nourishment of the foetus take 
their rise. The blood is returned from the foetus by other 
vessels which terminate in the placenta. These vessels, 
which unite the foetus with the placenta, are collected into 
the umbilical cord or Navel-string, so named. in consequence 
of the form exhibited, and the place of insertion in the foe- 
tus. At the commencement of this evolution, the os tincee 
is obstructed by a glairy matter, which, while it closes the 
aperture, prevents the adhesion of the margins. But, in 
order to illustrate this curious subject. still farther, it. will 
be necessary to return to the structure of the ovum. 
The ovum, after it has sufficiently increased in size, ap- 
pears to: be surrounded by two membranes or involucra. 
The innermost one is termed amnios, and the external one 
chorion. : 
The amnios is destined to secrete a peculiar fluid, term- 
ed liquor amnii, with which the foetus is. surrounded, This 
