INCUBATION, 



pulmonary vefle's could regularly tranfmit blood to the lungs, 

 and yet pot be vilible. 



Botli Hallcr and Maitre Jan agree that the liver makes its 

 appearance towards the end of the fourth day. It is feen as 

 a vifcous cloud of a reddiih yellow colour, fituated between 

 the umbilical veiiels and the defcendiiig aorta. It is render- 

 ed nioi-e vifible and opaque by moillening it with dilliiled 

 vinegar. 



The eye of the fcctus appears black, the choroid coat 

 being forrntd ; and the fuperior part of the eye cliicfly pre- 

 lenting itfelf to \\c\~, the pupil is fituated in the inferior 

 portion of the i ye. 



The tm! of the fifth Jay, or i2o hours, the vafcuhr area 

 is fo much extended as to cover generally more than half the 

 fuperficies of the yolk-bag. The trunks of the veffels are 

 proportionally increafed in fize, but the branches near the 

 circumference are not fo much advanced. Some additional 

 vefTels are feen to proceed from the umbilicus of the chick 

 towards the left fide of the area. Thefe a]>pcrtain to the ve- 

 ficula umbiiicalis, as will be better underllood hereafter. All 

 the blood-veffeis of the area, which were in the firft days of 

 incubation of a fine fcarlet, are now commonly found of a 

 dufky red colour. 



The air.iios continues to increafe in magnitude, and con- 

 tains fo much fluid, that the chick appears to be furrounded 

 by a green liquor. 



Tiie ch'ich, efpecially when excited by a little external heat, 

 can be perceived to move its limbs. 



When the external membrane is removed, the vcftcu'a 

 umhiUcalis is found to be confiderably larger than it was the 

 preceding day. It is likewife perceived to have contrafied 

 fome adhefion with the external membrane, which however is 

 undone by a flight force, particularly after the membranes 

 have been a (horc time immerfed in vinegar or fpirits. 



The branches of blood-vefTels which are dillributed upon 

 the veficula, belong to a trunk that comes out through the 

 opening of the abdomen at the umbilicus of tiie chick. The 

 veficula is connefted with the fcEtus by means of a neck, or 

 contrafted part, which is likewife received into the abdo- 

 men at the umbilicus. 



M.iitre Jan defcribesat this period a numberof fine branches 

 of blood-ved'els upon the (kin of the chick : we have not, 

 however, obferved thefe fo early. 



When the internal furface of the yolk bag is examined, it is 

 found that the eminences upon it, already mentioned, have 

 affumed the form of veflels ramifying from the centre, where 

 the ch:ck is fituated, to the circumference of the vafcular 

 area. This appearance has been miilakcn by many of the 

 older anatoraifts for real veffels dillributed upon the inner 

 furface of the yolk-bag, and were fuppofed by fonie to be 

 defigned to convey tlic yolk to the chick. When the appear- 

 ance of the yellow veffels of the yolk-bag is firll prefented, 

 it is very difficult to detect the realocca(ion of it, but as it 

 advance.s, it becomes plain that the effect is produced by the 

 inflexion or doubling of the inner membrane of the yolk- 

 bag, ill the courfe of the real blood-veffels of the bag. By 

 a careful examination o: the matter w ith a lens, even at this pe- 

 riod, we have always perceived the .ippearance of yellow vellcls 

 to arite from the blood-veffels running in grooves, or refledlions 

 of tlie inner membrane. When the two tunics of the bag are 

 feparatcd, the blood-veffels are found to belong to the exter- 

 nal, upon the inner fide of which they appear as emineiit 

 lines ; wliile upon the external iide of the inner coat there are 

 correfponding grooves or Lmpreffions. 



The hear,' Al this period begins to undergo the changes by 

 which it approximates in ftruclure the heart of the adult 

 bird. It becomes perpendicular, or in the line of the foetus, 



Vol. XIX. 



from being placed in a tranfverfe direftion. The point is re- 

 curved between the two lobes of the liver. 



The auricles become more diflinft, and the interval be- 

 tween them white. 



The auricular canal is confiderably fliortcned. 



Thi ficond ot right ventricle begins to be forced before the 

 termination of the fifth day. It appears as a little fac under 

 the bulb of the aorta. It is much (horter than the original 

 ventricle, above which it is placed. Its figure is oval, and 

 it has a red colour. In a fliort time the new ventricle is feen 

 in its proper fituation, that is, on the riglit fide of the heart. 

 Haller Hates that he found it placed on tlie rigiit as early as 

 the 1 1 7th hour. There are tl.en two dillindt drops of blood 

 in this part of the heart of the foetus, and a white line is per- 

 ceived between them. 



The aorta arifes from the left ventricle. The branches, 

 which are furnifiied in order to form the dorfal aorta, have an 

 inclination downwards, and unite in an acute angle. At 

 this period alfo the bulb sf the aorta begins to diminifti, and 

 to retire into the flefli of the heart. 



The liver is more diilinft than at the end of the preceding 

 d.iy ; it is divided into lobes, and is of a pale or yellowifn 

 red co'our. Its veffels receive blood. 



Haller firfl perceived the redum at this period, although 

 the Itomach and other intellines were not yet formed. It ap- 

 pears in the (hape of a trident, the lateral proceffes of which 

 arc produced by the rudiments of the tv.-o cosca. 



The End of the 6lh Day, or 144 Hours. — Eggs placed fo 

 long under the hen are confiderably diniiniflied in weight, 

 which is even perceivable on taking them into the hand. The 

 facll begins to appear more dry, and opaque coloured, and is 

 more fr.igile than before incubation. 



The diminution of weight depends upon the enlargement 

 of the air-cell, which now extends over a confiderable part of 

 the great end of the egg. Miitre Jan has taken fome pains 

 to prove that the air contained in he e'g<g is not derived from 

 the atmofphere. He luppofed that the fluids filtered through 

 the inner layer of the membrane which lines the fhell, and 

 that by the a£lion of heat, they were volitili/.ed or 

 converted into air. This opinion is not only improbable, 

 but refuted by particular fafts. On opening the air-cell, we 

 do not find its furface to poffefs any moifture ; that layer of 

 the membrane which lies next the humours of the i<g%, al- 

 tliough lei's thick, is fmoother, denfer, and lefs porous than 

 the layer immediately in contaft with the (hell, and therefore 

 lefs fitted to admit the tranfpiration of either fluids or air. 

 The chemical properties of the gafeous contents of the air-cell 

 feein to put it beyond all doubt that the atmofphcric air paffcs 

 into the egg. (Seethe article Egg.) We rtial! only add, 

 that when we have varniflied the external furface of the flicll 

 the air-cell no longer increafed in fize. 



Upon opening an egg at this period of incubation, it. is 

 not unufual to fee the chick turned upon its back, inllead of 

 lying, as before, upon its left fide. This circumftance feems 

 to depend upon the amnios having fo much increafed in fizc 

 that the fuctus may roll round within it. 



Tiie membranes around the chick begin to acquire more 

 ftrcngthand firmnefs, which is the confequcncc of the vefi- 

 cula umbilicahs becoming more clofely and exttnfively • 

 united to the external membrane. 



The veficula is now found to be very much increafed in mag- 

 nitude, and to cover a confiderable portion of the volk-b'jg. 

 When the membranes furrounding it are cut open, and the 

 veficle expofed, its neck or peduncle may be traced as a fine 

 tranfpavent tube to the reiitum, by a cartful examination, 

 particularly if the parts have been rendered opaque, b)' 

 D being 



