INCUBATION. 



fubftance of the yolk, which is the origin of tlie bed of the 

 chick. We cannot determine whether this depredion con- 

 tains a fluid or not. The part of the white fpot which fiir- 

 roundi the deprcflion that contains the foetus, is occafioned 

 by the fiibftaiice of the yolk liaving a pecuUar organiza- 

 tion. At that point it is white, and has a curdy or 

 coaguhited appearance. Malpighi bcheved tliat lie could 

 difcern the foetus as early as the fixth hour, and even before 

 the egg had been fat upon ; but mod other obforvers have 

 not been able to fee any appearance, which they thought 

 deferved to be conlidcred as the fcetus, before the 1 2th hour. 

 It is, howevt-r, probable, that the embryo of the chick is 

 formed after impregnation, although from its extreme ten- 

 dernefs and obfcurity of figure, it cannot be detected until 

 it is a little developed by incubation. 



The amnios appears to be coeval with the chick. 



The rin^s or circles [ha/oncs) that furround the cicatri- 

 cula, are now fomesvhat enkrged, and more deiined in their 

 o\ithne. They alfo affume a httle of an oval figure. The 

 external circle is undulating, or forms wave?, through which 

 the colour of the yolk appears. Maitre Jan defcribes four 

 halones, Halleronly three; we have found the number van'. 

 This appearance of circles is not eafily accounted for. We 

 have thought it arolc from the feparation of the membranes 

 into their proper layers. 



F:r/l Day, or after 24 Hours. — Upon opening an egg at 

 this period, it is perceived that the fpace formed for the 

 admiflion of air between tlie layers of the external membrane 

 is a little more enlarged. 



The external '^kite appears fcarcely diminiflied ; it is fluid 

 and tranfparent, and runs out at tlie opening if the egg be 

 inclined to that fide. 



The yolk is found to float higher \n the principal white than 

 before incubation, and appears a httle flattened upon the 

 upper furface. 



The halones are more extended, particularly the outer one. 

 The internal one has acquired a little of a heart-lhape, and 

 is furrounded by a more defined line of a greenifli colour, 

 ivhich is the firll appearance of the outline of the veinous 

 area. There arc alfo lome faint and imperfecl traces of 

 green veiTels, or rather elongated points, to be dotecled in 

 different parts of the fpace inclofv;d by this line, which are 

 the rudiments of the veffels that are to compofe the area. 



The amnios is well formed, and compofed of two circles 

 ■which unite two ftraight lines, refembling the figure of a 

 peftle. 



The Jalus has a more determined figure ; it is larger at 

 both extremities than in the middle ; but the end which cor- 

 refponds to its head is larger than the other, and forms an 

 oval mafs. Maitre Jau Hates that if an egg, after being under 

 the hen for 24 hours, be put into water, it will float with the 

 fmall end downwards, which is a proof that a part of its 

 fluids is loft, and replaced by the admiflion of air into the 

 air-cell at the great end of the egg. 



After 36 hours we have found the greenifli line, already 

 mentioned, to have Hill more plainly the appearance of a 

 veflitl. Ii encompalTes a larger area, and afl"umes more com- 

 pletely the contour of a heart. The greenifli points alfo 

 now are continued into each, and put on the appearance of 

 veflels, which are moll numerous next the circumference of 

 the area, but prefent no evident conneclion with the foetus. 

 Some of thefe veiTels are fecn to contain red blood, more par- 

 ticularly thofe which pafs between the head of the chick 

 and the upper part of the veflel in fliape of a heart. In fome 

 cgg'» a- this period, we have found thefe red veflels approach 

 the chick but not join it :• in others they were coUefted by 

 a finglc branch, wliich palTcd behind the head of the chick. 



This branch Levellle has called the meningo-cardiac vein, 

 and others the vena afcendens. There is confiderable va- 

 riety with refpeft to the period at which red veflels begin 

 to appear. We have feen them before the 36th hour, but 

 oftener, perhaps, not before the 40th, or even later. Authors 

 differ extremely on this point. Malpighi reprefents the 

 commencep'.ent of the veffels at the 30th hour (Append, 

 figs. 18 and Tf). ), and even delineates them at the 12th hour. 

 (Epift. 1. fig. 5.) Stcnon obferved them at the yiil hour ; 

 Vefling at the 43d, but f iw the points at the 42d hour. 

 Aldrovandus found them the third day. Maitre Jan de- 

 fcribed the firft appearance of veflels at the 38th hour, 

 and reprefented them dillinftly formed, and carrving ret! 

 blood at the 44th hour. (P'. 53 and 58, and fig. 10.) 

 Haller noticed lome reddidi points at the 36th hour, and 

 to the 48th, but did not perceive perfect red veflels before 

 the end of the third day. Memoire 2d fur la Formation 

 du Cccur dans le Poulet, &c. p. 23. 



Thefe contradictory accounts depend in fome degree upon 

 their authors not having fufficiently attended to the exiilciict? 

 of colourlefs points and veffels before the formation of red 

 blood. 



Haller ftates, that he obferved, at 31^ hours, the head of 

 the fatiis to have the appearance "of being fplit, which he 

 attributed to the vcficles of the brain having a fmall degree 

 of opacity, wUilll the membranes compofing the cranium 

 were fo tranfparent as to be feen through. Malpighi alii) 

 reprcfented this appearance. We have not noticed it. 



Haller likewile defcribes the head of the chick to begin 

 to form at a right angle with the tail at the 40th hour. 



After 48 hours, or tzvo days, the external halones occupy 

 a confiderable extent upon the furface of the yolk, and be- 

 come fainter in their appearance- 



The liquid or external luUle is fomewhat diminiflied. 

 The principal -while begins to fubfide to the bottom and 

 fmall end of the egg. 



The yolk-bag appears flatter upon the upper furface, and 

 its contents are a little more fluid than before incubation. 



The chalaza, wliich were originally fituated at the two 

 poles of the yolk-bag, are now found fomewhat below them,, 

 and confequeiitly rather nearer to each other. 



The -vafcular area (figure veineufe of Haller) is now in ge- 

 neral completely formed with colourlefs and red veffels. The 

 number of the latter is very variable. We have found them 

 at this period in very different flates of advancement, but 

 always difcoverable by the greenifli veffels which precede 

 them. The area has very perfetlly the figure of a heart, 

 except that the part correfponding to the bafis is formed by 

 the outline of the two fides being confiderably curved before 

 their junftion in the middle, at which place the outline of 

 the area forms a pointed projection downwards, which 

 nearly reaches to the head of the foetus. It is this part of 

 the area which receives the branches of the meningo-cardiac 

 vein already mentioned. The veffels are now alfo conneded 

 with the chick by means of their principal trunks. 



The pulfalion of the heart is evident at this period, and 

 even in fome hours before, according to Maitre Jan's ob- 

 fervations. It appears to be produced by the alternate 

 conti-aftion and dilatation of two or fometimes three veficles. 

 This appearance is the punelum fa/iens of ihc older anatomifls, 

 and has been commonly, but erroiieoufly, reputed the firil 

 action of life. 



One mode of preparing eggs for examination is to remove 

 a portiM^f the membranes including the foetus and vafcular 

 figure around it, and having waftied oft' any pieces of the yolk 

 that may adhere to them, immerfe them for a fliort time 

 in Ihong fpirils or dillillcd vhiegar, by wliicli means th» 



pulpy 



