INCUBATION. 



fuppofcd that the fhell was brolcen by the hen, in which opi- 

 nion Haller fvems to concur. He ftates tliat he found a (iffure 

 in the lliell, and the mciiibranes which inclofcd the chick, at 

 the fame time entire. Maitre J;in, on the contrary, afcrihes 

 the fraciure of the fhell to the chick. He found the beak had 

 lacerated the membranes befere the fhell was broken ; and 

 having watched the eggs at the time the chicks began to 

 crv, he did not perceive the hen touch them with her bill, 

 but that the fhell was ruptured from the infide, and the 

 fractured portions were pulhed outwards. This account of 

 the mode in which the fhell is opened, exaftly accords with 

 our obfcrrarions. We have fecn the fhell evidently broken 

 from within ; beiidcs, the fuel of birds being hatched by arti- 

 ficial heat, proves decidedly, that the afliflance of the hen is 

 not neceffary for the frafture of the egg. 



After the fhell is broken the chick is always heard to cry, 

 and the point of its beak is feen to projcft out of the egg 

 for feveral hours befoi-e the chick attempts its liberation. 



It is reported by common people who have the care of 

 fitting hens, and it is even afferted by fome anatomifts, that 

 the cries of the chick are heard before there is any aperture 

 made in the fhell. Haller relates that he took an egg, in 

 which he difcovered the piping of the chick, and which was 

 at the fame time unbroken, and having carefully removed the 

 ftiell, piece by piece, he found that the opaque membrane, 

 ■which Hues the fhell, was alfo entire, but that there was an 

 opening in the chorion oppofite to the beak of the chick. 

 He does not ftate whether the bill had penetrated into the 

 air-cell, but it moll probably did, as at this period the two 

 layers of the membrane hning the fliell are feparated beyond 

 the fituation of the beak of the chick. In another obferva- 

 tion Haller found the chorion entire under the iilTure in the 

 fhell, which was on the oppofite fide of the egg to the bill of 

 the chick; and therefore he fuppofes the (hell to have been 

 cracked by the hen. We cannot pofitively contradict the 

 alTertion of the chick uttering found before it has the means 

 of freely refpiring, but we fliould conceive it extremely im- 

 probable; and upon every occafion where we have heard the 

 piping of the chick ilTue from the egg, we have found that 

 tliere was a crack or frafture in the fliell. 



Where the chorion is lacerated by the beak of the chick 

 there is always fome blood fhed, but the edges of the mem- 

 brane foon dr)-, which prevents any farther hjemorrhage ; for 

 the circulation is not yet interrupted in the chorion by the 

 contraftion of the fphinfter at the umbilicus, although Maitre 

 Jan defcribes the paiTage of the yolk, and the deftruclion of 

 the chorion, as events of the fame date. 



When incubation has proceeded regularly, the whole of 

 the yolk-bag at this period is found within the cavity of the 

 abdomen, which is clofed by the contraftion of the circular 

 fafciculus, of mufcular fibres placed around the umbihcus; 

 but, as already mentioned, this does not, until near the exclu- 

 iion of the chick, comprefs the umbilical veffels, fo as to de- 

 ftroy their circulation. 



The ingurgitation of the yolk caufcs a great tumefatlion 

 of the belly of the chick. The animal refembles one in a 

 gravid Hate, but the belly is lefs fwoUen than might have 

 been fnppofed from the bulk of the yolk-bag. The latter 

 does not lie in the front of the cavity, but is prefTed in be- 

 tween all the vifcera, and therefore occupies the leall pof- 

 iible fpacc. 



It is remarked by Maitre Jan, that more chicks die about 

 this period than at any other of incubation, which he attri- 

 butcb to the dillention of the abdomen by the yolk-bag ; 

 but we fhould fnppofe that it is the ceffation of the external 

 circulation upon the membrane of the chorion, which caufes 

 the great fatality towards the end of incubation. 

 7 



Maitre Jan ftates that he difcovered the fluid of the yofi' 

 at the 484th hour mixed with the contents of the fmall in- 

 teftines ; but fome other anatomiils, who admit the paflage . 

 of the yolk through ^he ductus vitello intefiinahs, do not 

 pretend to have detefted it in the inteftine fo early. 



The f/'/Vi', about this period, is fametimes obferved tore- 

 main for a whi'e perfeftly motionlefs. Harvey fuppofed 

 that it was afleep on thefe occafions. How far he was cor- 

 reft we cannot pretend to fay. 



According to Haller the weight of the yolk is not fcn- 

 fibly diminifhed before its entrance into the belly of the 

 chick. He weighed the entire yolk-bag at the joodth hour, 

 and found it to be three drachms and two grains. He flates 

 that the weight of the yolk, before incubation, was three 

 drachms and ten grains. This diminution of weight is, 

 therefore, not greater than might have been espefted frojB 

 evaporation, or from an original difference in the bulk of. 

 the two eggs that were the fubjecls of the experiment. 



Haller, in feeking for the allantcis, or -vefctda timlilicalh, at 

 this period, ftates, that he could net diftinguifh it from 

 the chorion, or rather from a fine membrane he raifed from 

 the internal furface of the chorion by inflation ; but he found 

 a little fac in the form of a pear, extremely vafcular, full of a 

 yellow mucus, and terminated by a peduncle which was not an 

 inch long. We have already defcribed the veficula umbilJ- 

 calis as being loft in the formation of the chorion long before 

 the prefent period. Malpighi, likewife, fuppofed this part 

 to have the fame deftinalion in the latter periods of incu- 

 bation : and in none of our obfervaiions have we met with 

 the pear-fhaped bag mentioned by Haller. We have turned 

 "back the chorion at the prefent date, and have found it to 

 be a perfect fac, fuch as we have previoufly defcribed it, 

 and connefted with the adjoining membranes by refleSiors 

 of a very delicate texture. Thefe were readily detached, 

 and left the chorion pendent by its vefTcls from the umbi- 

 licus of the chick. Befides the blood-vcffels, we could di(- 

 cover a tranfparent filament pafling to the inverted bag, whic ii 

 appeared to be tubular, and perhaps was the uracluis. 



Tie End of 2 I Days, or the full Period of the Jncubntion 



of the Hen When the progrefs of incubation has been 



regular, the chicks are now generally found to have aban- 

 doned the eggs, leaving the fhells with the fhreds of the 

 chorion and amnios behind them. It happens, however, 

 occafionally, that fome of the chicks have been backward 

 in their growth, from the unfavourable pofition of the eggs 

 in the nert, or the negligence of the hen. Thefe are fomc- 

 times found about this period within the broken fhell, having 

 the remains of the membranes ftill attached to the umbi- 

 licus. 



The reparation of the membranes docs not appear to be 

 effefted by a procefs of flough or of ulceration. They 

 appear rather to be torn off, as fome flireds and a fhort 

 portion of the trunks of the blood-vefTels are feen to pro- 

 ject at the umbilicus. The hemorrhage from thefe veffels 

 alfo is not entirely fuppreffed by the conflridtion of the 

 fphinfter mufcle, but in a degree by the afTiftance of a clot 

 of blood, which forms in the ends of the torn veffels. 



We have "not been able to obferve whether the complete 

 fradlure of the fhell, previous to the exclufion of the ch:ck» 

 and the detachment of the membranes, are acconiphfhed by 

 the efforts of the chick alone, or with the affiftancc of 

 the hen. 



The belly of the chick, after exclufion, is much enlarged, 

 though lefs tumid than it was when the yolk-bag was firfl 

 taken into the abdomen. It would, thertfore, appear that 

 the yolk begins to diminifh immediately after it is admitted 

 into the belly of the cliick, although fome obfervations of 



Haller 



