INCUBATION. 



/pulpy fubftance that compofes the chick is rendered opaque 

 ■ and vifible, and the ilrufture of the membranes more dif- 

 -tinguilhable. 



If tliis Itep be taken with the egg at the prefent period, 

 ■or even before it, a fine glofly tranfparent membrane will 

 be found upon the external furface of the yolk-bag, which 



The vafcular arta is now generally found to be perfefled 

 with red velFels. The contour of the area is more extenfive, 

 particularly in the tranverfc direttion. The diameter being 

 now about an inch and a quarter, it has therefore lefs the 

 fliape of a heart : the pulfation of tlie three veficles, and 

 diftribution of the blood-veflels, are very clearly feen through 



very readily detached, leaving the foetus and the vafcular the tranfparent membrane which covers them. 



The amnios begins to affume the figure that it is after- 

 wards to poiTefs. T!ie two circles which encompafs the 

 head and the tail of tlie fatus become enlarged, principally 

 on the fore-part, or the fide next the right-hand of the ob- 

 fcrver ; but the fac is a little contracted on the fame lide 

 oppotite to the umbilicus of the chick, thus tending, in a 

 degree, to give the amnios the form of a kidney. 



The fatus has a curved pofition forwards, and its body 

 appears larger in proportion to its head, than on the pre- 

 ceding day. 



We have forborne to defcribe particularly the heart and 

 llood-veJfcU of the chick and area until the prefent period, 

 when they are more diftinclly feen. 



In order to difcern pcrfcdlly thefe parts, the membrane 



including the foetus and vafcular area (laould be removed from 



the yolk-bag, floated in clear water, and examined with a 



lens. The vafcular area contains four or five principal 



branches, which ramify upon the membrane, and 



beneath. This membrane, we conceive, is the fame 

 which, llrengthened by the veficila umbilicalis, becomes the 

 one which at lall entirely envelopes the contents of the incu- 

 bated egg. 



The membrane, containing the vnfcular aira at the fecond 

 day, may be dillinguifhed to be conipofed of two layers ; 

 the external is fmooth, and receives the veflels ; the internal 

 .is a little thickened and fpongy. The remaining portion of 

 the yolk-bag is not feparable into diiferent layers. From 

 lience it would appear that the divifion into layers is the 

 confequence of veiTels being diilributed to the membrane. 



The amnios is now found a little larger, and the tranf- 

 parent fluid with which it is filled in greater quantity. The 

 fuperior circle alfo, or that which inclofes the head of the 

 chick, is larger than the lower. 



The futus has altered its fhape a little. The head has 

 acquired fomething the figure of the ace of clubs, the an- 

 terior prominence of which correfponds to the nofe and 

 mouth ; and the two otiigr projections are the velicles con- 

 taining the brain in a very foft pellucid ftate. Maitre Jan produce a very clofe and intricate net-work towards the cir- 

 ftates that the eves alfo are vifible at this period. (P. 77.) curafcrencc of the area. Thefe branches appear, when the 

 Haller did not fee the eyes before the 5 1 R hour, at which chick is in the natural fituation, to be lo many dUbna trunks. 



Two of them are feen to come from the fides of the chick, 

 ther parts of the head, by their'promin'ences a third from under the tail, and one, or fometimes two, 



^ branches from under the head of the chick. The two firft 



oommonly called the vmbilical veflels, as they come out 



time they had not any colour, and were merely diftinguilh 



able from th " 



and fituation. Seft. sii. Second Memoir 



The /jf^ir/, before tl;e chick is removed from the membranes 



and rendered opaque by immerfion in fpirits, appears to be of the chick at the umbilicus : they would, perhaps, with 



ituated on the outfide of the body, and the pulfating points 

 feem unconnefted w th each other ; but upon a clofe exami- 

 nation it is feen that the heart is not uncovered, although, 

 from its projection, and the natural tranfparency of the 

 parietes of the chetl, it had every appearance of being fo. 

 The heart altogether refembles a horfe-fhoe in form, or a 

 portion of a circle, of which the convexity is turned for- 

 wards, and the limbs backwards, one being directed to th 



more propriety, be called mefenteric veins, as we have much 

 reafon for fuppofing that they are t'je fame which are fo 

 plainly branches of the mefenteric vein in the latter ftages of 

 incubation. At the prefent period they terminate in the 

 vein which correfponds to the vena cava and vena ports. 

 Both the branches which emerge from undei the tail and 

 head of the foetus are found to be given off from the umbi- 

 lical veins jult ouifide the body of the chick. This origin is 



upper part 



of the cheit, and the other to the lower. The concealed from view until the lower furface of the membranes 



heart at this time coniills of an auricular canal, a ventricle 

 and a bulbous enlargement belonging to the aorta. The twc 

 iall are the falieiit points, and when the chick is fo far ad 

 vanced that the auricle is formed and pulfates, which is fome. 

 times the caic at the 4Sth hour, there are three falient points 



nd chick are carefully examined. Several anatomifts feem 

 to have publifhed errors with refpeft to the fuperior or 

 afcending vcfiels of the vafcular area. They are often de- 

 fcribcd as diftintt branches, which are lull on each fide of 

 the notch or chink, which has been already mentioned to 



be feen. Maitre Jan defcribes four falient points, and exilt during the firll days of incubation in the contour of 



fuppofes that they are the rudime;its lof the two ventricles 



and two auricles of the heart. At other times, he fays he 



has fo;;ietimes feen but three pulfating points, or perhaps 



two, or only one. Haller, who paid particular attention to Malpighi, Maitre Jan, Haller 



the developement of the heart of the chick, contradicts branches always unite into 



the vafcular area. They were, therefore, fometimes fuppofed 



to be feparate vefTels. Langly reprefented them to be at 



firrt diilindt branches, which united at the 6il'c hour; but 



&c. ftate that thefe two 



behind the head of the 



Maitre Jan's account. At the 4Sth hour he could difcover foetus w 

 but two falient points ; at the joth hour he faw three, but 

 on no occafion more than that number. He fuppofes that 

 when there arc two falient points the auricle does not pulfate, 

 and when only one the bulb of the aorta is without motion. 

 Me.noire !. p. S3. 



The End of the third Dny, or Seventy-two Hours. — This is 

 a very important period in tlie hiilory of incubation. 



The reparation of the layers of the external membranes 

 for the admifTion of air continues to increafe. 



The ha/ones are fcarcely to be difcovcred. The yoU is ever, reprefented it as being compofed of an intcrtexture, or 

 more extended, and the white continues to graviiate to the plexus of the ultimate branches of the veins which are fpread 

 lower parts of the egg, over the vafcular figure. There appears to be fome founda- 



tioQ 



they do not fooner. Malpig!ii, Maitre Jan, and 

 others, however, rcpref nt this veflel as coming from the 

 heart : Maitre Jan even fuppofes it may be the afcending 

 aorta. Our obfervations, neverthelefs, lead us to agree with 

 Haller refpedting this veflel, who ftates it to be a branch of 

 'the umbilical vein. It is this veflel to which Leveille has 

 given, as we conceive improperly, the name oi meningo-cardiac 



The veflel which forms the contour of the vafcular area 

 has been commonly defcribed as a fingle vein : Malpiglu, how- 



