INCUBATION. 



nation. The right iliac artery is often a mere fiiament, and 

 in fome in/lances it feems to be waiitintf altorretlicr. 

 . The venous branches which accoiripany the uiroilical ar- 

 teries are fo fmall that they are hardly diilinguiihvble, al- 

 though the former are of fufficient magnitude to be injected, 

 and have a vifible pulfation. Tliere is, tlierefore, a quantity 

 of blood fcnt to this membrane which is not returned to the 

 fixtus. The colour of the blood, botli in the umbilical ar- 

 ■ teries and veins, is a purple. 



The vafcular membrane, above defcribed, has received 

 different names. By the older anatomies it was ufualiy 

 called the chorion, to which perliaps it is moll analogous. 

 Hallei- gave it the name of the umbilical membrane, and Le- 

 veille has lately called it the hag-foaptd membrane (membrane 

 faccifcrme), from tlie circin:nilance of its envc'oping all the 

 parts of the egg. We have hitherto called it the external 

 meinbrnnc; until it fi:ould be defcribed ; but hei-eaftcr we 

 fliall fpeak of it as the chorion. In the account of the unin- 

 cubated egg, the membrane, on which the fhel! is moulded, is 

 called theexteniai, wliich it (Iridiy is. It fliould not be con- 

 founded with the chorion, from which it differs fo much in 

 figure, ftrufture, and ufe. See Egg. 



Haller appears to have not diftinguifhed the chorion from 

 the membrane of the yolk-bag, which contains the vafcular 

 aica. The progrefs of thefe two membranes is exceedingly 

 difl'erent, and the fydem of vefTels belonging to each is 

 perfectly didincl. The valvular area is confined to the 

 furface of the yolk-bag, and is intimately connefted with 

 its membranes, and the valvular apparatus already defcribed. 

 The veffels of the area are chiefly venous, and are formed 

 independently of the heart of the chick. They are mod 

 numerous in.the ear'y periods of incubation, although they 

 increafe in fize along with the other parts of the chick. 

 They go to terrhinatcin a large vein which proceeds directly 

 to the auricle of the heart in the firrt days, but, after the 

 liver appears, is feen to pafs behind it, and when the inteflines 

 are forraedMs found upon the mefentery, and conlUtutes the 

 largeft b;-anch of the vena porta:. 



The arteries of the vafcular area are infignificant, until 

 towards the end of incubation. When firll perceived, they 

 are given ofT from the dorfal aorta, and after the deve!o])e- 

 nicnt of tjie intcftines they are difcovcred to be branches 

 of the mefenterlc artery. The vcifels dillribuled to the 

 Srea appear, therefore, analogous to the omphalo-mefentcric 

 arteries of quadrupeds, although theii- deflination and func- 

 tions may differ. 



The -oiife's of the chorion, as we have already faid, are 

 branches, or rather, in the fcetal ilate, the trunks of the 

 iliacs extended upon this membrane. They do not exift pre- 

 vious to the appearance of the veficula umbilicalis, with 

 the growth of which they keep pace. The arteries of the 

 cliorion exceed the fize of its venis in the fame proportion 

 as ths veins of the yolk-bag exceed the arteries of that pait. 

 The vafcular area, therefore, collefts and tranfmits the 

 b'ood to the fcctus, while the arteries of the chorion difpofe 

 of a great portion of it in an extraneous circulation, and iu 

 the formation ofa thick and e.Ktenlive membrane. 



Halier and otheishave defcribed, at this date, an appear- 

 ance of white veffels upon the albumen. Maitre .Ian recki ns 

 feven of thefe white lines or veffels. They arifc from a little 

 circle which is in the centre of the white ; they divide into 

 branches in advancing towards the vafcular circle of the yolk- 

 bag, which forms at this time the boundary of the feptum 

 between the yolk and the white. Maitre Jan defcnbes 

 . them as paffr.g beyond the feptum and extending to the 

 yolk. Hullcr conjectures that the uL- of Uiefe veifels ir.ay 



bo to abforb the white and convey it into liic yolk. Obferv, 

 226. Mem. I. 



We have not made fuch an examination of thefe white 

 lines, as to enable us to pronounce whether they are .vefi'els 

 or not, or what their ufe may be. 



The 'water of the amnios appears to diminifh about this 

 period of incubation. It alfo becomes gradually iefs pure. 

 It is Hated by Maitre Jan, that when the fluid of the am- 

 nios is boiled at this date, the greater portion of it acquires 

 the hardnefs that the white of "the egg poflefies after being 

 boiled, although previous to the experiment it is much more 

 fluid than the albumen ; from wiience Maitre Jan conjectures 

 that fome of the fluid of the white is conveyed into the fac 

 of the amnios, but he does not pretend lo explain the means 

 by which this iseiFecled. 



The curd-Uhe fubjlance, already mentioned as being found 

 in the Komach and intefthies, is now met with alio in the 

 crop and ocfophagus. According to Haller, boihng water 

 makes this fubftance hard in the fame manner as it does the 

 white of an egg. 



The end of 15 days, or 360 hours, the membrane of the 

 chorion continues to increafe in llrength, and its arteries 

 in fize. Haller fpcaks of having fometimes found the latter 

 of a lighter red than the vein.s, which does not accord with 

 our oiifervations. We have generally perceived the arteries- 

 and veins to be limilar in colour, or at leail when a difference^ 

 did cxi:l it was immaterial, and fometimes the arteries, and 

 fometimes the veins were the lighter coloured velTels. The vef- 

 fels of the chorion have always appeared darker in our obfer- 

 vations than thofe of the yoik-bag, but the character of all 

 the blood is venous in the incubated egg during the latter 

 periods. The reflections of membrane which accompany 

 the principal veffels of the chorion, are now more firm than 

 before, and continuous with them appears a fine tranfparcnt 

 membrane that envehipes the yolk as far as the vafcular 

 circle on the edge of the white, and extends over p.-u-t of the 

 amnios, to which membrane it adheres, fo r.s to cover an. 

 irregular triar.gular portion of the chick in its amnios, and 

 inclofe more loofcly the im.bihcal /heath with the convolu- 

 tions of the intedines which proje£t from it and the neck of 

 the yolk-bag. This membrane does not appear to have beea 

 well undericood by the older anatomiits. 



They fometimes fpeak of it as a layer of the yolk-bag,, 

 and at others as the veficula umbilicalis, or allantois. The 

 belt defcription of it kas been given by Leveilie, who has alia 

 named it very appropriately entero-clorilyme, or tJje membraui 

 containing both intcfiines and yolk. 



The origin of this membrane is Hill involved in a degree oF 

 obfcurity. Leveille defcribed it when jjerfettly formed, but 

 has thrown no fight upon the hiflory of its growth. The 

 prefent date is the firil at which it lias appeared to us to be 

 perfected, but we conceive that it is one of the original 

 membranes and the fecond layer, that in the firfl days comes 

 off the vafcular area as a very fine pellicle.. It feems to 

 be COP- ■ °-ed, or rather continued into the peritoneum, lining_ 



the a„ 1 in the late periods of incubation, of which we 



fliall fee farther proofs hereafter.. There is no reafon for fup- 

 pofin_g this membrane to be the veficula umbilicalis, orallan- 

 toi.s, as the veficula is a fironger membrane in the middle of 

 incubation than this membrane is fevcral days afterwards, 

 when it is completely formed.. The proper contents of the 

 allantois alfo are found, throughout mcubation, within the 

 cavity of the chorion, to the formation of which we have 

 fuppofed that bag conlributcs, but never under the membrane 

 that inclofes the intelline and yolk-bag.. 



Haller Hates that he examined the -jalves upon the inner 

 fiirface of the ycli-Lrg with a lens, and djitiiidtly faw the 



iatcfliuifjurm' 



