INCUBATION. 



*ith the veficula umbilicalis ; J the veficula, the neck of 

 which i> feen to enter the opening at the umbilicus of the 

 chick. 



Fig. 6. is a magnified view of the heart of the chick as 

 it appears when hrft formed ; a indicates the horfe-fhoe 

 dilatation mentioned in the defcription of that organ on the 

 fecond day. 



Fig. 7. fhews the heart, and large veffels of the chick 

 magnified after thefe parts are more perfeAly formed ; a is 

 the vena cava ; i the auricle ; c the auricular canal ; d the 

 fingle ventricle ; e the bulb of the aorta ; f the principal 

 artery. 



Fig. 8. is a view, alfo magnified, of the heart, pulmonary 

 arteries, lungs, and aorta of the chick ; a is the heart feen 

 on the pofterior fide ; 6, b, the two arteries which fend off 

 the pulmonary arteries, and afterwards concur towards the 

 formation of the abdominal aorta. Thefe veifels, after part- 

 ing with their branches to the lungs, are analogous to the 

 dudlus arteriofus of mammalia 5 c, c, are the pulmonary 

 branches of the veffels lad mentioned ; d the third vefTel 

 of the heart, which correfponds to the fuperior ■part of the 

 aorta ; e the abdominal aorta, formed as already defcribed ; 

 fyf, the two lungs. 



Fig. 9. is a view of the internal furface of the yolk-bag 

 after the valves have begun to form upon it. 



Fig. 10. (hews a fimilar view of the internal membrane 

 of the yolk-bag, when the valvular apparatus is completely 

 formed ; a, a, a, indicate fome of the inteftiniform tubes upon 

 the edges of the valves ; i, b, the appearance of ved'els in 

 the intervals between the valves. The valvular projeftions 

 themfelves are fufEciently evident. 



Fig. II. (hews a portion of one of the valves of the 

 yolk-bag magnified ; a the bafis of the valve, where the 

 principal veffel runs ; b the plaits or folds upon its fide ; c 

 the inteftiniform tubes upon tlie edge of the valve. 



Fig. 12. is a magnified view of a portion of a \tilv<; of 

 the yolk-bag after it had been converted into a fine lace, 

 the veffels injeiSled, and the internal membrane wafaed, fo 

 that the real blood-veffels alone remain ; a is the principal 

 veffel that runs along the bafe of the valve ; b the one which 

 goes along the free edge ; c the lace-like ramification of 

 veffels between thefe two. 



Fig. 13. reprefents the yolk-bag, and the portion of the 

 inteftine with which it is conncdled, taken out of the chick 

 a few days after incubation ; a is the yolk-bag very much 

 reduced in fize, and of an ii-regular form ; b the piece of 

 inteftine to which tlie yolk-bag is attached ; c the duftus 

 vitello inteftinahs imperfeftly feen, as it is partly covered 

 by the blood-veffels of the yolk-bag ; d the branch of the 

 mefenteric vein, which furni(hes, in the early periods of 

 incubation, the veins of the vafcular area ; e the branch of 

 the mefenteric artery, which goes to the yolk-bag. It di- 

 vides, paffes on each fide of the inteftine, and unites again 

 before it ramifies upon the yolk-bag. 



Fig. 14. (hews the chick after the yolk-bag had been 

 taken into the belly ; a the umbihcal opening furrounding 

 the aperture, which is clofed, in the prefent inftance, by 

 the contraction of this mufcle ; b fome lacerated veffels, 

 and a portion of the chorion hanging from the umbilical 

 aperture ; c the belly of the chick diftendcd in confequence 

 of the admiffion of the yolk-bag. 



Fig. 15. is a plan of the relative pofition and magnitude 

 of the contents of the egg about the ninth day of incuba- 

 tion. The view is procured by firft boiling the entire egg, 

 and afterwards making an equal fedion ; a the edge of the 

 fhell ; b the cavity left between the two layers of the mem- 

 brane that lines tlie (hell to contain air. This cavity is 



confiderable at the prefent period ; e the chick in its am- 

 nios ; d the yolk ; e the cut edge of the yolk-bag ; / 

 the white which has defcended to tlie fmall end of the egg 

 and diminiihed ; g the chaiazat, much nearer each other 

 than they were before incubation. 



Fig. 16. exhibits a view of the contents of the egg, 

 with the chorion cut open and turned down off the other 

 parts ; a tiie chick feen obfcurely, as it is in fome meafure 

 covered by the membrane that includes the yolk-bag and 

 intellines, befides being furrounded by the amnios, the fluid 

 of which, at the prefent period is greeni(h ; i^the amnios 

 feen at each end of the chick ; c indicates tlie yolk-bag, 

 which is divided into three lobes ; one of tliem is feen to lie 

 upon the fide of the chick ; the blood-veffels cf the yolk ap- 

 pear fmall, and are indillinftly feen; ^ fome folds of the mem- 

 brane which invelopes the yolk-bag, inteftines, and a part of 

 the chick ; e the white contained in its proper membrane, 

 and very much reduced in fize ; y the chorion turned down, 

 and therefore the furface that is feen is the internal ; 

 gigigi refledlionsof iine membrane wliich attach the chorion 

 to the other membranes; A, />, the two trunks of the umbi- 

 hcjJ arteries, which each divide into two branches, and are 

 diftributed to the chorion. 



Fig.- 17. is intended to explain the formation of that 

 membrane which includes the yolk bag and the inteftines 

 of the chick, and for this purpofe the chorion is iiitirely 

 removed ; a is the chick contained in the amnios ; b indi- 

 cates the membrane of the yolk-bag and inteftines ; it is 

 feen to cover a certain extent of the amnios and chick ; 

 it defcends pofteriorly to be connefted to the feptum, be- 

 tween the yolk-bag and the white : anteriorly it is laid 

 open, in order to turn out th? yolk-bag, which was before 

 contained in this membrane. Within the chink made by the 

 incilion are to be feen the inteftines of the chick perfeftly 

 uncovered and defignated by ,:. The yolk-bag hangs by the 

 duclus vitello inteftinalis and its blood-veffels, and is marked 

 by the letter d. 



Fig. 18. (hews an egg with the great end fradlured pre- 

 vious to the exclufion of the chick : fome more pieces of 

 the fliell are removed in order to bring into view the 

 membranes that are lacerated by the bill of the chick ; a is 

 that portion of the membrane under the (hell which forms 

 the air-cell, and in which there is a fmall aperture. The 

 chorion and amnios are torn to a greater extent to permit 

 the bill of the cliick to appear on the outfide of the (hell ; 

 b the bill of the chick : befidc it are feen fome points of fea- 

 thers projecting out of the opening made in the outer 

 membrane. 



Fig. 19. is a portion of the fmall inteftine taken from the 

 adult bird, on which the appendix is placed, that we have 

 defcribed as the remnant of the duftus vitello inteftinalis ; 

 a the inteftine ; b the appendix or cscum ; f is a little 

 knot which was, during foetal life, the yolk-bag. 



In fom.e of the preceding figures, the form of the chick 

 is reprefented at different ages, which the reader will under- 

 ftand without the letters of reference. 



Of ihs Ufes of the different Parts of the Egg during Incuba- 

 tion, and aft^r the E>.dufion of the Chick. — Although there 

 can be no doubt that the chick derives its nutriment from 

 the humours of the egg, the mode in which this is effefted 

 is involved in confiderable obfcurity. Anatomifts have 

 entertained very different opinions upon the fubjeft. Some 

 have fuppofed that both in mammiferous animals and 

 birds, the foetus is nourilhed by fwallowing or abforbing tlie 

 hquor amnii. 



Heifter, in his Compendium of Anatomy, ftates, that after 

 the contents of the uterus of the covi were frozen, he found 



a piece 



