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, 



^^ (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 Laviell6. 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 

 albmnen 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 Levielle, 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 ; and 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 portse 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 Levielle 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. 



