198 OVIPARA. 
plan on which they are constructed, is greater than. that of any one of 
them with the Mammalia. 
Oviparous generation essentiallyconsists in this circumstance—that the 
young animal is not attached through the medium of a placenta to the pa- 
rietes of the uterus, or of the oviduct, but remains separated from it by 
its most external envelope. Its aliment is prepared before hand, and en- 
closed in a sac attached to its intestinal canal; this is what is called the 
vitellus, or yolk of egg, of which the young animal is a sort of appendage, 
at first imperceptible, which is nourished and augmented by absorbing the 
fluid of that yolk(a). Such of the Ovipara as breathe with lungs have, 
k=" (a) It is from the inner surface of the membrane of this vitellus, that a nou- 
rishing fluid is absorbed, this fluid being transmitted by a peculiar duct to the intes- 
tinal canal. The following facts regarding the eggs of birds have been collected by 
Dr. Burns of Glasgow, whose researches we have, in several instances, repeated, and 
found to be correct. As the following account embraces the various species of eggs, 
it may be convenient for us to remind the reader that the three classes of Oviparous 
Vertebrata animals consist of—1. The Birds; 2. The Reptiles; and 3. The Fishes. 
“In the eggs of fowls, we observe the following circumstances. Upon removing 
the porous shell, we find the albumen enclosed in a membrane, consisting of two 
layers, and called sacciform by Laviellé. These are separated from each other at 
the large end of the shell, so as to form a small sac, called the folliculus aeres. The 
albumen is divided into three strata; the first, or cortical, is most liquid; the second 
or middle, is more abundant, and thicker than the first, but less so than the third or 
central. The middle and central strata are enclosed in a delicate membrane, called 
leucilyme by Leviellé, which separates them from the cortical. Within the albumen 
we have the vitellus or yolk, which is enclosed in a vascular membrane, called chlo- 
rilyme, or membrana vitelli, which again is enveloped by a membrane common to it 
and the intestines of the chick, called entro-chlorilyme. To each end of the vitellus, 
we have connected a portion of the central albumen, called chalaza; aud in each of 
these a membranous substance is discovered, attached to the membrane of the 
vitellus, and a vascular structure, which can absorb the albumen into the vitellus, to 
contribute to the nutrition of the chick. Upon the vitellus, we observe the cicatri- 
cula, or small sac, called by Harvey the eye of the egg, and which was supposed to 
contain the foetus, the rudiments of which are allowed by Malpighi, Haller, and 
Spallanzani, to be pre-existent to fecundation. This cicatricula was considered as 
analogous to the amnion, and supposed to contain a transparent fluid, called by 
Harvey colliquamentum candidum, or liquor amnii. More modern observations 
ascertain that the embryo is not formed in the cicatricula, but very near it on the 
vitellus, and that the amnion enclosing it can at first scarcely be distinguished from 
the embryo. ‘The cicatricula soon disappears. Harvey’s account must therefore be 
transferred to the amnion. During incubation, the vitellus becomes specifically 
lighter than the albumen, and rises towards the folliculus aeris. Two arteries and 
two veins go from the meseraic and hypogastric vessels of the foetus, to the mem- 
brane of the yolk, and are supposed to absorb the vitellus, which therefore is carried 
to the vena portze of the chick, and nourishes the foetus. There is also a connexion 
betwixt the intestines and the vitelline membrane, by means of a ligamentous sub- 
stance, which was supposed by Haller and Vicq. D’Azyr to be a tube, and called 
vitello-intestinal canal, for it is said that air has been passed through it. It was 
supposed to absorb the yolk, by many villi on the inner surface of the vitelline mem- 
brane; but these are said by Leviellé not to be vessels, but soft lamellated plates. 
At the end of the second day, red blood is observed on the membrana vitelli, A 
series of dots are formed, which are converted first into grooves, and then into ves- 
sels, which go to the foetus. This appearance has been called figura venosa, and the 
marginal vessel, vena terminalis. The vitello intestinal ligament, and these vessels, 
form an umbilical cord. But besides these, we find, after the fourth day, a vascular 
membrane at the umbilicus, called membrana umbilicalis, which rapidly increases, 
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