INCUBATION. 



nally contained in the egg occupy a much larger fpace, as 

 already mentioned, than when the chick is fully formed. 



The preceding experiments prove that the air does not 

 pafs into the egg for the purpofe of efFefting any change in 

 the blood of the fcetal chick, fimilar to the procefs of refpi- 

 ration, as has been generally fuppofed. It is true, ncver- 

 thelefs, that when the blood is firll expofed to the air con- 

 tained in the great end of the egg, it becomes oxydated by 

 it ; and hence, during the firfl: days of incubation, all the 

 blood-veflels of the vafcular area are obferved to be of an 

 arterial colour : but as there are no means of expelling the 

 air as it becomes fpoiled, and as the fupply of pure air after- 

 wards is too fmall in quantity to produce any effect upon 

 the blood, all the veflels are of the purple or venous colour 

 in the latter periods of incubation. This appearance of the 

 blood has been obferved to exift by many %vriters upon in- 

 cubation, and coincides with the experiments we have re- 

 lated above. 



It may be aflced, what end is anfwered by the diftribution 

 of fo much blood to the membranes of the egg, if it be not 

 for its expofure to oxygen gas ? This is a queftion wliich, 

 perhaps, cannot be folved by direft experiments. 



We conceive, however, that it is neceffary, during foetal 

 life, to have a greater quantity of blood formed than is em- 

 ployed in the organs of the young animal. The funftions 

 of the blood-vefTels of the fcetus are almoft confined to the 

 creation of its body ; but in the adult animal the principal 

 part of the mafs of blood is confumed in furnifliing the va- 

 rious fecretions. The vafcular membranes of the ovum may 

 be confidered as performing the office of the various fecretory 

 organs, the actions of which are dormant, at lead during 

 the early periods of foetal life. By this means alfo the or- 

 gans of the body are accuftomed to the preparation and 

 prefence of a quantity of blood which is wanted immediately 

 after the young animal leaves the uterus or the egg : 

 indeed, without fuch a proviCon, it would feem to be 

 impoffible for the foetus to aflume, in a moment, the func- 

 tions of an independent ftate of exiftence. 



Further, the extraneous circulation on the membranes ap- 

 pears to be the means by which the nutritive fluids of the 

 egg are converted into blood. This is almoft demonftrated 

 in the vafcular area, in which the red blood appears before 

 its veflels have any vifible conneftion with the body of the 

 chick. 



The membrane containing the vafcular area, therefore, we 

 deem the organ of nutriment and affimilation. It corre- 

 fponds to the different vifcera which convert the food into 

 blood in the full-grown or perfect animal. The chorion 

 of birds fupplies the place of the vifcera, which return the 

 particles of the adult body to the common ftock of nature. 



We could draw many arguments in fupport of this theory 

 from the anatomy of the lower claffes of animals, and from 

 the ftrufture of the vafcular fyftem in particular animals of 

 a higher rank ; but it does not feem confiftent, with the 

 nature of the prcfcnt article, to enter into an extenfive phy- 

 fiological difcuiTion. 



If it be admitted that the blood of the foetus does not re- 

 ceive that efieft from the air whicli gives the arterial cha- 

 rafter, it becomes delirable to afcertain the power of the 

 foetus for generating animal heat. With this view we have 

 inftitutcd the following experiments. 



Ex. 1. — An egg, on which the hen had fat i; days, was 

 placed upon a bath of mercury heated to 104 of Fahrenheit, 

 and kept at that temperature for feveral hours. The heat 

 of the room was 75' . The upper furfaceof the membranes, 

 when the fhell was broken to admit the thermometer, raifcd 

 he iHftrument to 92 . The interior of the egg was 95', and 



on the inflrument being thruft down fo as to touch that part 

 of the fhell in contaft with the mercury, it did not rife liight-r 

 than 100°. From this experiment, the ftandard heat of the 

 chick appeared to be about 95^. 



Ex. 2. — An egg, at the fame period of incubation as tin- 

 lafl, had the lliell entirely removed, and was placed upon a 

 mercurial bath at the heat of S3", the air of the apartment 

 being 75". The centre of the egg preferved the inftrumoni 

 at 8S , and when pufhed down upon the membranes next the 

 mercury, it did not fall below 85". 



Ex 7, . — A chick, about 1 8 days old, was removed from the 

 egg and its membranes, and expofed for fome time upon 

 the furface of a bath of quickfilver at the temperature of 

 79", and becoming gradually cooler as the heat of the fur- 

 rounding air was 75 . The thermometer was introduce J 

 into the thorax of the chick, and flood at So" for a fhorc 

 time, even after the chick had died. 



The heat of the different parts of the incubated egg can- 

 not be expefted to correfpond with each other as luiu 

 as external heat or cold is applied to one furface o: 

 but the three laft-mentioned experiments, notwithtlanc. 

 appear to us conclufire, with refpecf to the foetal chick ft - 

 fefiing an independent temperature, and being capable of 

 reiifling, to a certain extent, both foreign heat and cold. 



It is proper to obferve, that we have made lom.e experi- 

 ments with refpeCl to the oxydation of the blood, and the 

 animal heat of the foetus in mammaha, which entirely cor- 

 refpond in their refults with thofe above related ; and, there- 

 fore, fortify our opinion with regard to the abfence of any 

 procefs analogous to refpiration during foetal exiftence, and 

 likewife prove that the young animal has an independent 

 temperature. 



We have already mentioned that the yolk lofes very 

 little, if any, of its original weight during incubation ; 

 but that after the chick is excluded from the fhell it is 

 fpeedily confumed. This humour, therefore, is obvioufly 

 defigned for a provifioiial nutriment to fuftain the chick 

 until it is able to procure a fufficient quantity of its natural 

 food. This difpofition of the yolk is particularly necef- 

 fary to thofe birds that live in a wild ftate, but is ufeful 

 to all. 



Anatomitls are divided with refpeft to the manner in 

 which the yolk is conveyed into the fyifera of the chick. 

 Some affert that it pafles into the alimentary canal, through 

 the duftus viteilo inteftinalis, where it is digefted previous to 

 its being abforbed by the lafteals. Others deny that it is 

 to be found in the ftomach or inteftines, and believe that 

 it is abforbed immediately from the yolk-bag. We are 

 difpofed to adopit the latter opinion, both from never having 

 being able to difcover any yolk in the inteftinal canal, and 

 from a number of reafons, which indireftly go to difprove 

 the patTage of the yolk through the duftus viteilo intefti- 

 nalis. 



In the firft place, there does not feem to be any power 

 by vvhiih the yolk can be tranfmitted into the inteftine. 

 The yolk-bag itfclf has no mufcular coat, and the mere 

 preffure of the parietes of the abdomen of the chick, even 

 if fufiiciently ftrong, would be exerted as much upon the 

 inteftines as the yolk-bag, and might therefore as readily 

 urge the contents of the former into the latter, as the yolk 

 into the gut ; for the duAus viteilo inteftinalis does not 

 penetrate the coats of the inteftine obliquely, fo as to pro- 

 duce the effeft of a valve, but has merely a flight contraftion 

 at tlie orifice, which muft operate equally on both fides. 



There would appear to be no neceffity for tlie yolk to 

 pafs into the alimentary canal, or to be fubjecled to any 

 procefs of digcftion or afTimilation previous to its entering 



the 



