1821.] Dr. Paris on the Physiology of the Egg. S55 



The parts of which the perfect egg consists are : 1. Vitellus, 

 or yelk, with its capsule and cicatricu la; 2. Albumina, with their 

 proper membranes ; 3. Chalazce ; 4. Folliculus aeris ; 5. Com" 

 mon membranes; 6. Exterior involucrumj or shell; to each of 

 which I shall successively direct my attention. 



The vitellus, or yelk, is the part formed in vitellario, and is a 

 yellow fluid contained in a membranous capsule, on which a 

 greyish-white circular disk is discernible ;^ this is named cicatri- 

 cula, and is the speck of entity, the germ that is to be developed 

 into the animal. " In hujas gratiam," says Malpighi, ^^ reliqua 

 comproducta videntur." We have here then arrived at the ear- 

 liest stao-e in which we can detect the existence of the embryon. 

 Our imperfect faculties will not enable us to ascend further, and 

 yet, even now, the body is formed as the experiments and obser- 

 vations of Malpighi and Buffon most satisfactorily testify. The yelk 

 is surrounded by a more tenacious fluid, of a light-straw colour, to 

 which the name albumen, or more commonly the white, has been 

 assigned ; this may easily be divided into two separate and dis- 

 tinct portions, each of which is contained in a concentric 

 /membrane. They differ from each other considerably in specific 

 gravity, and seem to answer different purposes in the economy 

 of the egg : the consistence of that w^iich is exterior is far less 

 than the one which immediately envelops the yelk, and is con- 

 sumed in the earlier periods of incubation ; f while the internal 

 and more viscid albumen seems reserved for the latter stages, 

 when the chick must require a greater proportion of generative 

 matter than at any other period of its evolution. 



Many of the ancient philosophers imagined that the chick was 

 formed out of the yelk, and that the white afforded nutriment. 

 Such a theory, however, must be at once abandoned, when it is 

 known that the vitellus suffers no other change by incubation 

 than a degree of liquefaction, and that it is drawn up into the 

 small intestines of the animal,;}: by means of an appropriate duct § 

 (ductis stenonis) just before its exclusion. It is then evident tha^ 



* Fabricius supposed it to be a vestige of tlie ruptured pcduncuhis^ or that portion 

 of membrane by which each yelk is connected to the vltellarium ; and Emilias Parisa- 

 nus contends that it is the semen of the male. 



+ There is a considerable difference discoverable in the milk of mammiferous animals 

 at different periods subsequent to parturition. Fourcroy ascertained that it is most 

 charged with calcareous phosphates immediately after parturition, and that the propor- 

 tion of them gradually diminishes. 



X The vitellus appears to be consumed in the first 10 days after the animal is 

 hatch ed . — IMonro. 



§ Mr, JMacartney observes, that the duct by v/hich the yelk communicates with the 

 intestine of the chick does not become entirely obliterated, but leaves a small sac, which 

 remains during the life of the animal. In the snipe, this appendage is of considerable 

 bulk, and on examining its internal structure, we shall find that it is lined with a kind 

 of villous coat, and that it has numerous folds, or projections, which indicate that it 

 possesses a glandular structure, exhibiting a curious example of the economy of nature 

 in adapting an organ of foetal life to the exercise of a particular function in the full' 

 grown bird. . 



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