INCUBATION. 



the fluid of the vcficula and the allantois as the nutri- 

 t of the fcEtiis during the iirll periods of its exiitence. 

 •imerriiigand BUmienbachfuppofcthat thevcficnlallmbili- 

 ^ IS intended for the nourifhmont of the fcetiis in the hn- 

 lubj^ft. Although it does not appear that this ufe of the 

 ' Kula or allantois lias been in any degree proved, the argu- 

 ments employed arc conclufive againlt it being a refervoir for 

 urine,- and particularly fo with refpeft to birds and the liuman 

 fubjea. 



The veficula umbfiicalis attains a confiderable fize, and is 

 filled with its proper fluid before the kidnies cxiil ; and 

 although the membrane continues to increafe, the quantity of 

 fhe fluid it contains by no means keeps pace with the growth 

 of the kidnies. \v'e cannot admit tlic ivafoning of Haller, 

 ^^'ho concludes that the exiftcn>.v> of fluid in tlie allantois is 

 a proof that the kidnies fecrete urine before they are vifi- 

 ble. It has not yet been afcertained that the fluid contained 

 ill the allantois is urine even in quadrupeds, 'r'lerethis bag 

 communicates fo plainly with the bladder. Lobllcin found, 

 after the allantois had acquired a very confiderable bulk, that 

 the kidnies were fmall and pale, and refemblcd organs which 

 had not yet been called into aftion. The bladder alfo was 

 contradled. He very jnc'.icioufly remarks, that if the allan- 

 tois were diftended to the fi/.e that it is known to poffefs in 

 the early periods of geltation and incubation by the fecre- 

 tion of the kidnies, it would become before birtli a moit 

 prodigious fac. It is extremely improbable that the fecre- 

 tion of urine fliould dimiuifli or be fufpended after the kid- 

 nies were perfedly formed, by which fuppofition alone the 

 fmaller relative iize of the allantois in the latter periods of 

 foetal life could be explained 



We may further obferve, that there is no refervoir provided 

 for any of the other fecretions of the fostiis, and iliat even 

 the gall-bag is not remarkably dillended, although it evi- 

 dently contains bile fome time before birth, and notwith- 

 itanding the liver is the largelt gland in the body. 



In defcribing the progrefs of incubation, we have mention- 

 ed, that the veficula umbilicalis of birds contributes to 

 thi> formation of the membrane called the chorion ; that its 

 fluid dilappears, leaving behind it fome layers of a foft cal- 

 careous fubflance, refembling the excrement of birds, or 

 rather the folid parts of the urine of thofe animals. This 

 exorementitious matter has been coTifidered a proof of tlie 

 veficula being the receptacle of the urine, but it fliould be 

 obferved that the fame fort of fubftance is found after the 

 confumption of all the fluids of the egg. 



The fuiiftions of the veficula umbilicalis in birds, if there 

 be any peculiar to it, are lofl; in thofe of the chorion. 



By recurring to the hi'.tory of incubation, it will be found 

 that the external membrane of the yolk-bag, on which the 

 vafcular area is fpread, and the chorion, receive a vaflly 

 larger quantity of blood than circulates at any one moment in 

 the body of the chick. The chorion is not originally a vaf- 

 cular membrane ; its vefTels arife with the veficula umbilicalis, 

 by which they are condufted from the body of the foetus. 

 The vafcularity of the chorion in fome degree fucceeds that 

 of the vafcular area ; it appertains chiefly to the arterial 

 fyftem, whilft that of the area is almoft entirely produced by 

 teins. 



The ufe mod commonly afcribed to thefe two great fyftems 

 of extraneous veflels, is the oxydation of the blood of the 

 fcetus, for which purpofe it is alio believed that the air is ad- 

 mitted into the great end of the egg. We have, however, 

 been led to entertain a different opinion of the offices of the 

 Vafcular area, chorion, and air-cell, in confcqiience of fome 

 experiments we have made, which we fliall lay before the 

 reader. 



Experiment I. — Two eggs of a hni were opened at the 

 great end by deilroying the (hell and outer layer of the 

 membrane compofing the air-cell. The blood-veflbls were 

 feen through the membrane of a deep purple colour. 

 Being expofed to the atmofpheric air, they became, in a 

 few minutes, a vivid fcarlet. The fame eggs were then placed 

 in a jar filled with fixed air, and their blood very foon re- 

 afl'umed its original purple colour ; and upon the eggs being 

 again brought into the common air, the fcarlet colour of the 

 blood was fpecdily revived. One of thefe eggs had its 

 blood rendered venous and the other arterial, in the manner 

 above-mentioned. Both were then placed in a vefl'el of water, 



they remained all night, and on the next 



morning 



each continued to preferve its proper colour. 



Ex. 2. — We took an egg which had been fat upon i8 

 days, and having removed a fquare portion of the (hell and 

 membrane forming the air-cell, about a quarter of an inch in 

 fize, it was placed in ajar of carbonic acid gas, over a bath 

 of quickfilver, which was kept throughout the experiment, 

 as nearly as poffible, at the temperature of 104 Fahrenheit. 

 The motions of the chick caufe an egg to roll upon a fmootii ■ 

 furface, but in this inftanc^ no motion wns perceived. After 

 the egg had remained an hour in the fixed air it was taken 

 out, and the aperture of the flicll was carefully clofed witk 

 paper and glue, and two hours elapfed before the eeg began 

 to roll upon the furface of the mercury. Upon this occa- 

 fion, therefore, the chick appeared to have been fubmitted 

 three hours to the influence of carbonic acid gas without- 

 being deftroyed. 



E^\ 7,. — An egg, at the iSth day of incubation, was 

 deprived of the greatefl; part of the (hell, and of the mem- 

 brane that lines the (hell, fo that the velfels of the chorion 

 were fully expofed. The membrane of the chorion was 

 moiftened to increafe the effeft. The egg was fuftered to 

 remain for 25 minutes in the fixed air, during which time 

 the chick (hewed no motion ; but upon being removed into 

 atmofpheric air ifftruggled as ufual. 



Ex. 4. — The air contained in the cells, during the latter 

 ftages of incubation, was collected from a number of eggs, 

 and fubjefted to experiment in a graduated glafs tube. It 

 afforded, as nearly as may be, the fame produdls that are 

 obtained from atmofpheric air, after it has been fpoilcd by 

 relpiration ; from which it would appear, that it was the 

 prtfence of undiluted fixed air in the 2d and 3d experi- 

 ments that caufed the motions of the chick to ceafe, and 

 not the ab fence of oxygen gas 



Ex. 5 — Being aware of the fatal effeifts upon the 

 chick, which have been reported to follow covering the 

 fhell of the egg, fo as to llbp the further admiffion of air 

 into it, we varniihed the furface of feveral eggs ac different 

 periods of incubation. The refult of this experiment was 

 nearly the fame in every inllance. The chick always died ; 

 not, however, immediately, but in two, three, or four days, 

 and apparently not in confequeiice of being uiifupplied with 

 frclh air, but from being unprovided with any kind of air 

 to occupy the fpace caiifed by the expenditure of the fluid 

 contents of the egg in the formation of the chick : for, in 

 thefe cafes, we found the yolk more or lefs diforganized, the 

 membranes fometimes ruptured, and the fcetus always mal- 

 formed or imperfcftly developed. If the death, of the chick 

 had been occafioned by the deprivation of oxygen, it would 

 have been immediate, and unattended with any change in the 

 flrufture of th.e foetus. 



The gradual admiflion of air into the eggs of birds takes 

 place as a matter of neceffity, becaufc the (hell, being hard and 

 unyielding, cannot accommodate its form to the bulk of its 

 contents in proportion as they dimini(h ; for the parts origi- 



naUy 



