Comparative.] ANA 
was granted to be in a very large quantity, and to agree 
in every property with that in the amnios, it would cer¬ 
tainly appear in the fame form for fome time in the ftomach; 
whereas it is always found very different there in the 
larger foetus ; and Harvey confeffes as much in this place : 
therefore it may be concluded, that it does not go down 
into the ftomach. If ever any thing like fames has been 
feen in the crop of chickens, as has been alleged by fome, 
it might be no more than the yellow or green-coloured 
fubftance brought up from the ftomach. 
Though creatures that refpire are under a neceflity of 
either fwallowing, or forcing back by vomiting, whatever 
is introduced beyond their fauces, it cannot be thence con¬ 
cluded that a foetus is under the fame necedity ; for, as it 
does not exercife refpiration, it will fuffer no inconveni¬ 
ence by a liquor lodging near to the glottis; whereas crea¬ 
tures that breathe cannot allow any fubftance to remain 
there without danger of the glottis being flopped, or of 
fuch fubftances falling down the trachea, either of which 
would be of bad confequence ; which the animal prevents, 
by forcing fuch fubftances out ol fuch a dangerous litua- 
tion. But, to enforce the negative of the colliquamentum 
■ pading by the mouth, obferve, that there are only three 
days in which this paffage can mod probably be fuppofed 
to happen, which are from the fifteenth to the eighteenth 
day of incubation : for before the fifteenth, the quantity 
of the liquor amnii is encreafing, which is no great fign of 
its being fwallowed ; and after the eighteenth, this liquor 
is not to be feen. If, then, the liquor amnii were all fwal¬ 
lowed between the fifteenth and eighteenth days, the fto- 
mach ought to be fuller at this time, and its contents 
fhould be thinner, more pellucid, &c. like to the colli¬ 
quamentum; which, however, does not happen. Befides, 
if we fuppofe the power of digeftion fo llrong as to ex¬ 
pel this liquor as faff as it is taken down in thefe three 
days, it would certainly follow, that this powerful digef¬ 
tion continuing in the three fucceeding days, while there 
is no liquor to be fwallowed, the ftomach ought to be quite 
emptied; which every one who opens the ftomachs of 
chickens at this time will fee it is not. And, laftly, as a 
more diredt proof dill againft Harvey, Dr. Monro broke 
the fltells of feveral incubated eggs, while the colliqua¬ 
mentum was in large quantity; and, before the amnios was 
opened, he faw the chickens open their mouths very wide 
feveral times, but could not obferve the quantity of the 
liquor in which they lay at all leflened. He afterwards 
carefully diftedfed the chickens, and found no other than 
the common fmall quantity in the crops, and the ordinary 
curdy mucus in the ftomach ; which feems to be a de- 
monftration that they do not fwallow. After Inch con¬ 
vincing proofs, we fliall only defire the reader to compare 
the polfure of a chick, and of a hen while fhe fwallows 
liquors, that they may fee the pofture of the chick’s neck 
to be mod unfavourable to the fuppofition of deglutition 
being performed ; and then (hall conclude with a very 
fliort hiftory of incubation, afligning the mod probable 
reafons of the feveral appearances. 
By the heat of the hen, aflifted poffibly by the action of 
the air contained in the folliculus aeris, the albumen be¬ 
comes thinner, efpecially where it is mod expofed to thefe 
forces ; and the vitellus in the fame manner becomes fpe- 
cifically lighter, and therefore readily rifes in the white. 
And as, by being divided into two unequal portions by its 
axis the chalazae, it prefents the fmaller portion to the in¬ 
cubating heat at firft ; fo the change in confequence of in¬ 
cubation being fooneft and mod produced here, and the 
cicatricula being enlarged at the fame time, the fmaller 
portion of the yolk becomes much lighter; and therefore 
is buoyed up to the fuperior part of the egg ; w hereby the 
folliculis aeris and the membranes of the cicatricula be¬ 
come contiguous when they enlarge, and the vitellus can 
never be in hazard of comprefling the tender embryo ; 
and the umbilical velfels are fituated fo as to have their 
extremities immerfed in the liquors that firft undergo the 
proper change, for being imbibed by their orifices. The 
Vol. I. No. 42. 
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incubation continuing, the white is ftill more and more 
colliquated, and the umbilical veflels are proportionally 
extended, the veins to abfor'o it, and the arteries to throw 
out any particles that are unfit for tiie chick till they are 
farther prepared. When the white in the tipper part of 
the egg is exhaufted, its membranes become contiguous to 
the amnios ; and thereby, the membranes involving the 
foetus, becomes fufficiently ftrong to refill the motions of 
the chick, when its eafie or fafety prompt it at any time to 
fpurn. The powers of incubation above-mentioned, aflift¬ 
ed by the pulfation and conquaflatory motions of the nu¬ 
merous umbilical velfels fpread on the yolk, diliblve that 
humour more, and render fome part of it fine enough to 
be taken up by the fmall extremities of the umbilical vein, 
fome of which penetrate its membrane: by which the li¬ 
quor at laft becomes thicker ; and the membrane, being in 
part emptied, will more eafily yield to the weight of the 
chick; and is prefled into the form of a horfe-ftioe, while 
the net-work of the velfels extended on this membrane 
renders it (tronger and firmer. The folliculis aeris not only 
aflifts in coiliquating the albumen, but, when the humours 
of the egg come to occupy a lefs fpace, by elcaping through 
the fliell, and by being changed into the folid fubftance of 
the chick, the folliculis enlarging, keeps the chick and 
humour fteady, without danger of being difordered and 
broken by the motions of the egg. Branches of the um¬ 
bilical velfels being diftributed to the amnios, the arteries 
will pour out their liquors into its cavity in greater quan¬ 
tity than the veins can take them up, as long as the foetus 
is weak: but, whenever the foetus becomes ftronger, and 
conlequently the ablot^ent power of the veins increafes, 
they will take up the fluid of the amnios falter than tire 
arteries pour it in, and its quantity will be diminilhed till 
it is quite exhaufted. This abforption will go on more 
fpeedily in proportion alfo to the umbilical velfels being 
lefs diftended with albumen, whereby there is lets refin¬ 
ance to the progreflive motion of the ablorbed liquors; 
which probably is the reafon of the colliquamentum being 
all taken up between the fifteenth and eighteenth days. 
By the conftant circulation and renewal of all thefe hu- 
mours of the egg, they keep frefh and uncorrupted in a 
fecundated egg ; but corrupt foon in a fubventaneous one, 
or in fuch whofe feetus dies in the time of incubation. 
Wherever velfels are not fufficiently filled, they contract 
themfelves; and therefore, the albumen being exhaufted 
in the laft days of incubation, the umbilical veflels gra¬ 
dually fhrivel, which prevents the danger of an haemorr¬ 
hage when the chick is feparated from its membranes. 
But, as the white is not fufficient at this time fully to fup- 
ply the chick, the yolk is taken into its body; and, being 
there prelfed, it is thrown gradually by the proper dudt 
into the guts to fupply that deleft. The veflels and glands 
which open into the alimentary tube feparate at leaft as 
much liquor as will moiften it; and the ftomach, having 
no callous ftrong cruft on its internal furface, will feparate 
more than it can do in the adult; and in the mean time 
the glands of the infundibulum pour out a liquor that is 
always thicker as the chick increafes, tiil it becomes a very 
thick white mucus : and therefore the contents of the fto¬ 
mach of the feetus in the egg muff have the appearance 
above defcribed, and will be (lowly parting oft into the 
inteftines. The fliell at the obtuie end of the egg be¬ 
coming more brittle, by being fo long expofed to a dry 
heat, and the membranes lofing their toughnels when their 
moifture is exhaufted, the chick very eafily tears them, 
and breaks oft'that end of the (hell, to make its way into 
the common atmofphere. The mother having no juices 
prepared within her body to give to the chick for food 
after it is hatched, and its organs for taking in and di- 
gefting aliment being for fome time too weak to fupply it 
fufficiently with nourifhment, the vitellus is made to fup¬ 
ply thefe deficiencies, till the chick is fufficiently confirm¬ 
ed and ftrong ; after which it is no longer the fubjeft of 
our prefent inquiry. We therefore proceed to finifh our 
anatomical obfervations on the domeftic fowls. 
8 G 
Thefe 
