INCUBATION. 



Haltr wtuld lead to a different conclunon. He weighed 

 the egg and its different contents at thr J26th hour, and 

 found the entire egg to be i ; drachms and as many grains ; 

 the weight of the yolk, with fome light membranes tiiat 

 adhered to it, was four drachms. The foetus weighed five 

 drachms and 14 grains. In an egg which had not been fat 

 upon, he found the weight of tlie entire egg to be to 

 drachms and three-fixths, and that of the yolk tliree drachms 

 and 10 grai'is. This account of the weight is obviouily 

 incorred. See Egg. 



Manv anatomies ftate that they have been able to deteft 

 the fhiid of the yolk in the inteftines at this period, and fome 

 fuppofed that tliey found it had pafTed into the alimentary 

 canal, even before the exclufion of the chick. JJallcr found 

 yolk in the inteftines at the end of the 2ni day, in one iii- 

 ftance where the chick had not left the egg. Maitre Jan 

 faw it after 20 days and fix hours. Needh^.m delcribes tlie 

 gradual paflage of the yolk into the inteiline at the end of 

 incubation. Langly aL^o found it in the iiilefline at the 

 fame time. Theod. Aides ftates that before the chick was 

 excluded, he could not prefs the fubftance of the yolk into 

 the gut, but that immediately afterwards he was able to do 

 fo. Stenon fucceeded in prefling the yolk through the 

 tluftus vitello inteliinalis. 



Notwiihiiauding the teftimony of fo many anatomills, we 

 are inclined to believe, that the fubftance of the yolk is not 

 conveyed into the inteiline. We have fcrupuloufly examined 

 the inteftines of chicks, both before and after the exclufion 

 from the egg, and in no inilance could we dilcover with cer- 

 tainty any yolk in them. In the la!l periods of incubation, 

 and until the chick has taken its natural food into the (lo- 

 mach, the alimentary canal is filled with a yellow fluid, which 

 it is fcarcely poffible to dillinguiih from the yoUc ; tliere- 

 fore, all direfi obfervations reipefting the paOage of the 

 yolk into the intcllines mull be attended with a degree of 

 doubt. There are, neverthelefs, many reafons befides ac- 

 tual examination ; for fuppofing that the yolk is Bot con- 

 veyed into the fyftem of the chick by palling firil into 

 the intellinal canal, which we (hall ftate in fpeaking of the 

 ufes of the different parts of the egg, we are further 

 fupported in our opinion by the authority of Blumenbach, 

 who afferts pofitively, that the yolk does not pafs into the 

 inteftine. We might alfo quote the fame opinion held by 

 LeveiUe ; but he advances it upon falfe grounds, as he de- 

 fcribes the duAus vitello inteftinahs as a hgament, by which 

 the yolk-bag is fufpended to the body of thc,cliick, notwith- 

 ftanding the tubular ftructure of this part is fo palpable. 



The fubftance of the yolk at the end of incubation is 

 extremely tenacious or gluey in its confiftence, and of a 

 deep green colour, particularly in the centre. 



The net-work of the valves upon the inner membrane is 

 very vifible at this period. ^Ve have thrown coloured fize 

 into the blood-vcffels of the yolk, and afterwards foftened 

 by maceration, and by frequent ablation detached the inner 

 membrane ; when we found that the bafis of the lace-like 

 or reticulated Itruclure of the valves was formed by real 

 blood-veffels, which prefented the moft beautiful vafcular 

 apparatus that we have ever feen. This mode of prepa- 

 Tation (hews, that the organization of the internal membrane 

 depends upon the growth of the blog4-ve(rels of the ex- 

 ternal membrane of the yolk-bag, and confirms the account 

 we have already given of the formation of the valves. Haller 

 conCdered the veffels diilributed in the valvular ilru<fture of 

 the bag as being chiefiy, if not enturely, venous ; but we 

 found them molt eafily injefted from the aorta of tiie chick. 

 In the early periods of iacubation we kcovv that the branches 



of the veins of the yolk-bag are larger and miirc, nwae.. 

 rous than thofe of the arteries, b\it we are inclined to 

 believe the arterial branches predominate towards the end 

 of incubation. Further obfervations, however, arc necef- 

 fary to explain the vacAdar fvflcm of this niombrjne. 



Several anatomills fpcak of a new membrane beiiig found 

 upon the yolk-bag towards the end of incubation, which. U 

 continuous with the (l<in of the chick. Haller, ni fouie.o|(- 

 fervations, delcribes this membrane in a manner which ieay«$ 

 no doubt of its being the fame that we have already no- 

 ticed as including the yolk-bag and intellincs of the chickj 

 when they are on the outlide of the body. 



We, as well as others, have found the appearance of a 

 new tranfparent membrane covering the yolk-bag after it haj 

 entered the abdomen of tlie chick. This membrane leeins 

 to be the continuation of the peritoneum. It adheres to, tlie 

 iniide of the umbilicus, and to that point of the yolk-bag 

 where the white and clialazsc were attached. We liaye not 

 had it in our power to invelUgate completely the connections 

 of the yolk-bag with the peritoneal. coat ot the parietca ot 

 the belly, but we believe at prel'ent that any appearance ol 

 3 third tunic to the bag is derived from the peritoneum. 



Haller ftates obfervations made upon the alLinlois or vf/i- 

 cula umLUlctiHs, even to the lall day of incubation. He men- 

 tions that he found under the chorion an extremely tine mem- 

 brane which was incorporated with the tiil'ue of white fila- 

 ments or ftreaks that we have fpoken of before. He 

 defcribes this membrane as giving an immediate covering to 

 the chick : it wa ; fo tender that inflation buril it. He fup- 

 pofes it to be either the amnios or the allantois. In the next 

 paragraph he fays, he fought the centre of the yolk, and 

 found the urachus there without having any connection with 

 the allantois. He inflated it, oiie part of the air went out 

 at the anus ; another elevated the membranes of the reilum. 

 He next paffed an extremely fine probe into the urachus, and 

 found an orifice on the left fide, between the anus and that 

 veficle that the urachus produced by inflation in the reftum : 

 the probe would not enter the plicated mufcular fac, which 

 is behind the rectum and at the fide of the vagina. 



This account appears to us to be very inaccurate in fome 

 points. The membrane which Haller took for the allantois 

 was moft probably that which envelopes the yolk-bag and 

 inteftines. The urachus at the periods of uicubation, when 

 it can be found, has no connection with the yolk-bag. The 

 foramen on the left fide of the anus appears to have been 

 made by the probe, and the mufcular fac he fpeaks of was 

 the burfa Fabricii. 



Changes -which take place in the Slrudurc of the Chick afkr 

 Incubation. — Thefe chiefly relate to the difappearance of the 

 yolk, and the degeneracy of , the arterial duc^s, which in the 

 fcetus conveyed the blood from the pulmonary artery into 

 the aorta. 



//; fwodays after cxchifion from the egg, \\\tytilk is very fen- 

 fibly diminiihed in quantity, and its confilleucc is even more 

 tenacious or gluey than it was before. 



The right du8us arteriofus, which is the firft obliterated, 

 becomes as fmall as a filament. The left is, according to 

 Halier, diminiihed three times the lize it had during incuba- 

 tion. 



In four Jays after exclufion, the joih is found to be re- 

 duced two-thirds of its bulk. In proportion as the fubftance 

 of the yolk difappears, the membranes forming the bag 

 alfo diminiih. The adhelion which we have meulioned.the 

 yolk-bag to have with the ii;ternal part of the umbjlicus, 

 after its admiffion into the abdomei>, is now undone. Jt 

 ■wouid appeaf that this attachment is produced by tjic con- 

 l' .^ ■ iradion 



