INCUBATION. 



g;reatcr in depdi; an alteration wliicVi arifes from the diminu- 

 tion of the white, and from the chalazx tending to ap- 

 proach each other, and thereby to bring the two poles of 

 t!ie yollc, at which thefe bodies were originally attached, to- 

 gether on the lower fiirface of the yolk.. The approxima- 

 tion of the chalazx feems likewife to occafion, in the firft 

 inllance, the divilion of the yolk-bag into lobes. At the 

 profcnt period the bag forms two lobes, one large and one 

 fmall. Tliefe afterwards, by the compreffion of the chick, 

 are ufually moulded i;)to three, but in the egg of fome other 

 birds into a greater number: thus the yolk-bag in the Guinea 

 jo'-jjl is divided into li\ or feven lobes. 



The End oj thi- \yh D,iy, or 312 Hours Upon opening 



an egg at this period, and removing the contents from the 

 Ihell, tlie tvbhi is found to have affumed a greenifh hue, 

 and to liave more contillopce than at any of the former 

 p.riods ; and it is inclofed entirely by a fine film, which 

 retains it in one pofition, and in llrict conneflion witii '.he 

 depreffion on tiie inferior furface of the yolk-bag- The 

 external vefTels are diitribnted over the whole white. 



The yolk-bag has, ft ill more of the lobulated figure, ar.d 

 the chick lies obliquely acrofs the bag in the fuperior dc- 

 prefiion between tlie two lobes. 



'I'lie valviihir llru(ftiu-e on the internal furface of the 

 volk-bag, which we have noticed before, has now arrived 

 at perfedion. The valves arc more eminent, form numerous 

 folds or coils, their edges appear to be covered by tubes 

 whicli are convoluted upon tiiemfelvcs in the manner of 

 intellines. Thefe have been called by Haller the I't-rmi- 

 Jorm and hiteJVnnform Ink's, and h;ive been defcribed both by 

 him and other anatomills as real vefiels : but they have al- 

 ways appeared, ii> our examinations, as reflections or pro- 

 celibs of the valvular ilruilure, in the fubllance of which 

 tlie real blood-velT.-ls ramify. Maitre Jan fpeaks of them 

 as vefiels which collect the fluid of the yolk, and which 

 afterwards terniinate in tlie blood-veficls. Haller confiders 

 the blood-vefi'els which run in the edge of the valves as a 

 tlillinft order of veffds, that end in the trunks of the veins of 

 the yolk-bag, and which take their origin at the circle, that 

 exifts in the latter days of incubation around the line wliere 

 the yolk and white are joined to each other. We have 

 already faid that the vefiels upon the edges of the valves 

 have numerous communications with thofe that run along 

 the roots or bafes of the valves, and which belong to the 

 external tunic of the yolk-baL--, and were originally the 

 fame veffels that formed the vafcular area. The venous 

 circle of the yolk-bag, we have no doubt, is the fame 

 that in the early llagcs of incubation has been defcribed 

 as the contour of the vafcular area : and the vermiform tubes 

 appear to be the villous or fpongy procefTes of the external 

 furface of the yolk-bag, which appeared in the firlt days 

 greatly extended, and grown into tiie form now defcribed. 



The c/jici, at the prefent period, has ftill more of its 

 perfed form ; its head is much curved, and the bill is placi-d 

 under the wing, 'whicii pofition is prefcrved throughout in- 

 cubation. 



The ii/l is pointed and has its natural figure. Haller 

 ftates, that tiie chick is often perceived to open and fiiut 

 the bill i-bout this period and afterwards. Maitre Jan, on the 

 contrary, although the chi^k moves its limbs frequently and 

 ilrongly, afferts, that it does not open its mouth. We have 

 not attended to this circumllance, from confidering it of 

 little confequcncc, and no way explaining the mode in which 

 the chick is nouriflied. 



Tiie itUtrnal organs of the chick are confiderahly advanced 

 to . irds perfcclion. The brain, although Itill foft and pulpy, 

 u well formed. Tlie tiilnies and tejes are completed in their 



figure. The jji/^cn refembles a conglobate gland. The /.■.•■> 

 are of greater fi/e than the heart. 



The firll branch that the aorta gives after its exit frim. 

 tlie heart is the left carotid, which gives off X.\\e fitbclainan ar- 

 tery of that fide. The trunk of the aorta then forms 

 an arch, which turns to the right. It feuds off the ri^ht 

 carotid, and approaches the vertebrje in paffing behind llie 

 rio-ht lung : it is united to the dudus arteriofus above the 

 liver. 



The right ventricle of the heart is Hill fiiortcr thin the left ; 

 it is of an oval figure, but the latter ends in a point. 



The end of the l^jh day, or 336 hours, the changes in the 

 humours and the formation of the organs of the chick are 

 found to have made progrefs fince the preceding day. 



We have already llated that the membrane which is ex- 

 ternal to the chick and the yolk-bag, and which is fcarcely 

 palpable in the firft days of incubatuni, gains llrengtli and 

 receives vefiels after the veficuia umbilica'is has acquired fome 

 fize, and has adhered to it. The magnitude of the vefiels, 

 and the thicknels of this membrane, incrcafe in propovtion as 

 the veficu'a umbilicalis grows : we therefore conceive that the 

 develope.nent of this membrane depends upon its connedior> 

 \vitli the veficuia and the b'ood-veirels which it derives from 

 thatbag. We have obferved, thatfome days beforeihe prefent 

 period, the externa! membrane begins tocxtend over thewhite, 

 which it uUialiy cnclofes about the 14th day : it therefore 

 forms a perfect fac, containing the chick in its amnios, the 

 yolk-bag, and the white. 



The veficuia umbilicalis about this period has arrived at a 

 very great i\/.e ; its extent cannot be eafily afcertained, as it 

 is fo much incorporated with the external membrane, that 

 they cannot be fcparated or accurately diftinguiihed : but w-e 

 fiiould conceive that the veficuia exceeds in fize the yolk-bag, 

 as tliere is the appearance of the external membrane being 

 thickened and ftrengthened by one under it as far as the 

 yolk extends. 



The manner in w^hich the veficuia concurs to form a mem,, 

 brane which in veils all the contents of the egg, appears to be 

 fimilar to the refledion of the'ferous membranes of the thorax 

 and abdomen, in order to cover the vifcera and lino the parietcs 

 of thefe cavities. It fiiould however be oblerved, that when 

 it is opened, there is no appearance of its forming an imme- 

 diate tuiiic iimilar to the pleura puhnonalis, but it feems to 

 be a fimpic bag, holding the contents of the egg, and only 

 united to them by fome refledions of very fine membran?, 

 which are chiefly attached to the back of the amnios, and in 

 the courfe of the trunks of the blood-vefPels. This circum- 

 itance has embarrafied other obfervers, and has led many 

 to overlook the ftiare that the velicula iimbil xalis has in tlie 

 formation of the common membrane of the parts of the 

 egg. 



The blood-veffils wiiich are dillributed to the external mem- 

 brane are, even at the prefent period, much larger tlian tlioi'e 

 of the yolk-bag. There are four long branches, wlilch fend 

 out numerous ramifications, that become fo minute as to evade 

 the fight ; but when this membrane is viewed with a mag- 

 nifying glafs, it prefents a beautiful fpedacle. The fituation 

 of the four great branches is not conliantly the fame ; as 

 tiiey arc difperfed upon a loofe membrane, winch varies iii 

 ■ fome degree its pofition according to circnmftances : one of 

 them, if not two, is always feen, however, to crofs the back 

 of the amnios. The four branches are arteries, and are 

 formed immediately on the outfide of the umbilicus of tiie 

 chick from the two iliac arteries; they, therefore, correfpond 

 to the umlilicd artcri s of ijiammiferous animals. The left 

 iliac artery is always of much greater iize than the right ; it 

 even appears larger than the aorta, of which it is the term;- 



