INCUBATION. 



traftion of the fphinfter mufcle upon the remnant of the 

 refTels and of tlic membrane* connefted with the centre of 

 the yolk-bag. 



It is now particularly eafy to fee the origin and courfe of 

 the proper bhod-vejfeh of the yolk. The veins belong to a 

 fingle trunk, which is continued along the mefentery, and 

 ends in the vena ports ; it is evidently a branch of the 

 tnefenteric vein. The arteries are alfo derived from a fingle 

 trunk, which is the principal branch of the mefenteric ar- 

 tery. This trunk, in fsme points of view, appears to be 

 double ; it divides, pafles on each fide of the inteftine, and 

 then unites to form a fingle vefTel previous to its dillribution 

 in the membranes of the yolk-bag. Thefe veffels, and the 

 duftus vitello inteflinalis, are connefted together by fome 

 fine folds of peritoneum, which are the continuation of the 

 mefentery. 



The cavity of the right duSus arteriofus is completely ob- 

 literated, but the left is fcarcely more diminifhed than it was 

 on the fecond day. 



In eight days after exclufton, the yolk is reduced to the 

 fize of a hazel nut. The duflus vitello inteflinalis becomes 

 thicker, and more refemblcs the inteftine than it did be- 

 fore. 



The valvular ftrufture upon the internal fiirface of the 

 yolk-bag is no longer apparent. 



The right dudus arteriofus, although reduced to the fize of 

 a filament, is enlarged where it enters the aorta. 



The left duS is hkewife obhterated at its origin from the 

 pulmonary artery ; it is a filament, but is inferiorly white, 

 opaque, and larger. Both thefe dufls, therefore, decline firft 

 at the fuperior part, where they come from the pulmonary 

 artery. 



The gizzard of the chick, efpecially if the animal has 

 had the opportunity of picking up fmall ftones with its 

 food, is thick and mufcular ; and the internal coat, which 

 during incubation was foft and fpongy, is now become firm 

 and dry. 



In i6 days after excliifwn, according to Haller, the fub- 

 flance of xhe yolk is entirely confumed ; there remains but a 

 fmall thin fac, lefs than a pea, containing a granular fiibftance, 

 which is partly of a calcareous nature, and which is the dregs 

 of the fubftance of the yolk. 



Twenty fex^en days after exclufton, Haller found both the 

 right duSus arteriofus, and the left, ftill remaining in the form 

 of filaments. The right was fmaller than the left. The 

 latter was ftill open at its junftion with the aorta. 



In three months after exclufton, the diiaus -vitello inteflinalis is 

 found to be turned down upon the fide of the inteftine, to 

 which it evidently forms an appendix. At its extremity may 

 ftill be feen the remnant of the yolk-bag, reduced to nearly 

 the fize of the head of a pin. It is ufually dark coloured, 

 from the nature of its contents. 



In the adult bird, Stenon, Ncedham, and Maitre Jan, have 

 recognized the ductus vitello intflinalis in the form of a fmall 

 c^cum. It is fituated on the alimentary canal, about midway 

 between the ftomach and the anus. 



Needham defcribes it as being half an inch long, but this 

 is incorreft with refpeft to any of the gallinaceous tribe, but 

 in fome other birds we have found it of a very remarkable 

 fize. 



In the fnipe it exceeds in length the caeca of the large in- 

 teftine. 



In the curlew it is about an inch long, and very capa- 

 cious. 



In the woodcoei it is nearly as large. 



It 18 above an inch long, and very (lender, in the tlaci 



Coot : the cseca of the great inteftines are alfo fingularly lon^ 

 and flender in this bird. 



In all the paflerine birds it is fmall. 



In theftvan, goofe, and duci it is of a moderate fize. 



It is remarkably fmall in the heron and in the kaiul. 



In all the birds we have examined, there exifts during life 

 fome remnant of the yolk-bag at the end of the cxcun., 

 formed by the duftus vitello inteftinalis. This is more par- 

 ticularly remarkable in the paffcrine and accipitrine birds. The 

 nightingale has a little fac, about the fize of a fmall pea, at- 

 tached to the end of this procefs of the inteftine. 



Although there is fo much variety with refpedl to the 

 figure and fize of the caecum of the fmall inteftines of birds, 

 the ftrufture of this part is conftantly the fame in every 

 fpecies. It appears to pofTefs but two coats ; one is the 

 continuation of the peritoneum, covering the inteftine ; the 

 other correfponds to the mucous membrane of the gut. The 

 internal furfacc of the Cfccum exhibits fmall cells fimilar to 

 thofe points of the alimentary canal on which the mucous 

 glands are congregated. This ftrufture exifts whether the 

 internal membrane of the inteftine be plicated or \-illous. 



We have not fonnd the appendix produced by the dnftus 

 intello inteftinalis to receive, in any inftance, the contents 

 of the alimentary canal. It is filled with a mucous fluid, 

 which appears to be fecreted from its internal fiirface. There 

 is, therefore, ever)' reafon to fiippofe that this appendix per- 

 forms the office of a mucous gland in the adult bird. The 

 great fize which it pofleffes in the fnipe, curlew, woodcoei^ 

 and coot is probably dependent upon the nature of the food 

 of thefe birds, although we cannot at prefent perceive in 

 what manner this part is fo particularly ufeful to them. See 

 Phil. Tranf. part ii. 1811. 



In the Ptet-defigned to illuftrate the incubation of the egg, 

 fig. I. exhibits an egg with a portion of the ftiell and the mem- 

 brane underneath it, removed at the great end : a is the firft ap- 

 pearance of the foetus as it is feen in the cicatricula, through 

 tlie membrane which covers it ; around it are the concentric 

 circles or waves (halones). 



Fig. 2. reprefents an egg opened in the fame manner in 

 order to expofe the foetus when it is a little more advanced, 

 with the commencement of the vafcular area ; a, the foetus 

 fomewhat formed with the head hke the top of a club ; i, the 

 contour of the vafcular area affuming the heart-ftiape : 

 it does not yet circulate red blood, but within it are 

 feen the rudiments of veflels containing a greenifti fluid. Some 

 of thefe points are of a darker colour, and are the firft ap- 

 pearance of the real blood. 



Fig. 3. is an egg alfo with the fiiell of the great end re- 

 moved ; a is the fo:tal chick ; i refers to the pulfating points 

 that conftitute the heart ; c reprefents the appearance of the 

 vafcular area after the circulation of blood is perfeited in it. 



Fig. 4. exhibits a magnifitd view of the portion of the 

 membranes at the great end of the egg, including the chick 

 and vafcular area about the fourth day of incubation ; a is 

 the thin gliftening membrane which readily feparates at this 

 period from the reft, ti:rned back ; ^is the fine tranfparent 

 membrane immediately under the firft, alfo turned a little 

 off^; c the chick contained in the amnios ; dthc veficula um- 

 bilicalis attached to the chick ; fthe external membrane of 

 the yolk-bag on which the vefTels compofing the vafcular area 

 are feen to fpread ; / is the internal pulpy membrane of the 

 yolk-bag which afterwards produces the valves, tlie pre- 

 ceding membrane being partially removed to expofe it. 



Fig. J. exhibits a portion of the yolk-bag witli the chick 

 and veficula umbilicalis confiderably magnified ; a the chick ; 

 6 the amnios ; c the membranous connedion the amnios ha» 



with 



