INCUBATION. 



insr, on account of their exhibiting in a palpable manner 

 the progrefs of fcttal cxiftence, that they have been lludicd 

 by almolt every refpcctable phyfiologiil. Writings have 

 been left upon this fubjeft by Ariftotle, Hippocrates, Fa- 

 isriciuS ab Aquaoendente, Aldrovandus, Coiter, Veiling, 

 Harvey, Langly,' Schrader, Theod. Aides, Stenon, Need- 

 iiam, Malpighi,' Maitre Jan, Haller, Hunter, Monroe, &c. 

 befides fome later obfervations by Llumenbach, Levcille, 

 and other continental anatomilU. Notwitliflanding the 

 labours of fo many eminent men, fome of thenioft important 

 points in the hittory of incubation remain ilill undetermined. 

 The beft treatifes arc thofe of Malpighi, Maitre Jan, and 

 Haller, but even in thefe authors there are many errors to 

 correft, and, except fome detached figures that have been 

 p'.ibhfhed to illuilrate particular parts of this fubjeCt, we do 

 not poITcfs any tolerably good plates. 



The deficiency and doubts that ftill reinain, with refpeA 

 to feveral parts of tlie hiilory of incubation, prove how 

 diflicult it is to invcftigate the fubjeft. 



The membranes of the egg, and the organs of the chick, 

 are in general fo tender, that they do not admit of being 

 eafily or fatisfaftorily examined ; and when prepared by 

 inimerfion in fpirits,' acids, or other aftringeiit liquors, 

 their natural appearances and connefiions arc much ob- 

 fcured. 



The progrefs in the growth of the membranes is found to 

 be in many circumftances unhkc what would be expefted, 

 and what it appears to be ; and the attachments of the mem- 

 branes to each other are extremely intricate and perplexing. 

 This part of the anatomy of the incubated egg, therefore, 

 principally requires improvement. In the following article 

 ve have endeavoured to fupply, by our own obfervations, 

 what was deficient in the hiilory of incubation, but, we mull 

 confefs, not with as much fuccefs as we could wifh ; there 

 are feveral anatomical points upon which we are obliged to 

 fpeak doubtfully, and we have not been able to fatisfy- our- 

 •felves rcfpeding the ufes attributed to fome of the parts of 

 the incubated egg, although we believe others will be found 

 to be elucidated by tlie experiments we have made, and which 

 are here related. 



Moll writers upon incubation, appear to us to have paid 

 too much regard to the dates or periods at which they have 

 obfcrved the different events of incubation to occur. Much 

 exaftnefs with refpeft to dates is not of material confequence, 

 and if it were, it is not prafticable, for the progrefs of in- 

 cubation depends fo much upon climate, feafon, and tlie 

 dihgence of the parent, with many accidental circumftances, 

 that hardly any two eggs agree precifely in their ftate of 

 growth, at any given period of incubation : we are not, there- 

 fore, to wonder at the dift"erence amongft writers upon this 

 fubjeft with refpcdt to chronology. 



The proper heat for the hatching of hens' eggs is 104° of 

 Fahrenheit, to which degree the furface of the body of the 

 hen will raife the thermometer when fhe fits upon her eggs. 

 In thofe birds that do not fit conftantly, but trufl to the 

 heat of the fun, the temperature of the eggs is probably /o//)V/c of Halli 

 below 104 degrees. 



The full period of incubation by the hen, in this countrv, 

 19 well known to be 21 days. In warmer climates it is faid 

 to be a day or two lefs. The periods of incubation vary 

 much in dilTerent fpecies of birds : we introduce the follow, 

 ing table, which has been compiled from different authors 

 by count Morozzo, in a letter from him to Laclede, to 

 fhew the periods of incubation compared with thJfc* of tlic 

 life of certaia birds. 



Crane and heron, as well , 

 by the heat of the fun. Jou 



ojlrich, hatch their eggs chiefly 

 . de Phyfique Ventos. an. 10. ' 



The ufual mode of exainining the progrefs of incubation, 

 is to gently frafture the fiiell, and pick it carefully off in 

 pieces with a p.iir of forceps. This fhoitld be done at the large 

 end, as it is there the changes of incubation commence, and 

 at all periods it will be found the moft convenient place for 

 breaking the fhell, on account of the air-cell being inter- 

 poled between it and the fluids of the egg. For particular 

 purpofes other modes of preparation mult be employed. If 

 it be our objeft to iludy the formation of the membranes, 

 thefe muft be feparated from the humours, and after being 

 cleanfed by being pafTed through feveral wtiters, th.ey are to 

 be floated in weak fpirits over a blue paper, or fome other 

 dark coloured fubftance. The ftrongeft fpirits, diftilled 

 vinegar, or a folution of alum, are neceffary to render the 

 organs of the foetus fufBciently plain for examination. The 

 only ufe of boiling the contents of the egg, is to fliew the 

 relative fituation and magnitude of different parts. 



In many obfervations, it is necelTary to magnify by glafTes 

 the fubjefts of inveftigation ; we would however recommend, 

 that the fight be only affifted by the fingle lens. When a 

 higher power is employed it is liable to deceive. 



After the egg has been fat upon for 12 hours, the quantity 

 of air contained between the two layers of the membrane 

 that hnes the (hell, is found to be a little increafed. 



No evident cliange is yet to be obferved with rcfpcft to 

 the bulk or cimfillence of the humours. 



The ivhtle fpot on the fupcrior furface of the yolk, which 

 contains the cicatricula of the ancient anatomifts, or tiie 

 is now fomewhat enlarged. There appears, 

 in its interior, a fmiill white body of an elongated and in- 

 diftiiift figure, which is furrounded by a line of a deep yel- 

 low or greenifii colour. If this part of the membrane be 

 removed and examined with a lens on the under furface, 

 after being a fhort time immerfed in fpirits or vinegar; it will 

 be difcovered that the elongated white body is the rudiment 

 of thc_/ar/uj, which is already inclofed in the itmnks. Tlic- 

 greenilh appearance would ieem to depend partly upon the 

 cxifteuce of the wnnios, and partly upon a deprclFion on the 



fubftance 



