INCUBATION. 



being a fhort time fteeped in vinegar or (Irong fpirits. Where have been dcfcribed in the preceding dars as contributing 

 it opens into the reftum it is dilated. to form the dorfal aorta. The right diiift is la-er in ap- 



The connexion that exids between the veficulaumbilica^is, pearing than the other, and hence, when the aorta is firit 

 and the end of the rectum, b)- means of a tube analogous to difcovered, it is obferved to be onlj- formed by two, branches. 

 the iirachus, has induced Haller and others to confider this About this period the liin^s have fiifSci'nt confidence to 

 bag as the refervoir of urine. be viiihle. ^Vhen they are rendered opaque by vinegar. 



We fhall find in tlie fubfequent periods of incubation, that they are found to be not to fmall, that they might not have 

 the veficula umbilicalis performs very important and other been feen fome time before if their tranfparency had per- 

 funftions than thofe of a refervoir of urine, in which, never- mitted it. Their figure is cylindric Miey are attached 

 thelcfs, it does not perhaps materially differ from the allantois polleriorly to the back, and connected with the two fides of j 

 of quadrupeds. the heart. : 



It is necefTary here to notice an error which two of the bell The Ussr has acquired more of its future form than it had 1 

 writers upon the fubjeft of incubation have fallen into with the preceding day. i 



refpcft to the veficula umbilicahs. Both Malpighi and The y/om.w/i is apparent at this time. It is very thin and 

 Maitre Jan fuppofcd it to be a part which afterwards be- tender, and refembles a good deal the forn of the human 

 came the ftomach of the chick. This miitake appears to ftomach ; its orifices are, however, nearer to each other, 

 have been occafioned by the veficula becoming, as we Ihall The llomach is received between the lobes of the liver, 

 afterwards defcribe, fo much extended as to lofe the figure of The fluid contained in the csrelral v^Jicks has acquired by 

 a bag, and from being united to other membranes, only to this time fome confidence. 



be diitinguifiied with difficulty. Its being wifible nearly three The /(■(V/a/V.f ai-e difcovered during this day as long rcddifli 

 days before the ftomach ; acquiring even then a much greater granulated lines or traces in the loin<;. 



ftze than that vitcus pofieOes, and differing effentially in The End of the "Jtli Day, or l6i Hours. — Upon opening art 

 figure and texture, fliould however have preveiited thefe two egg at this period, we perceive thac the changes already 

 parts from being confounded with each other. mentioned in the form, confidence, and fituation of the hu- 



The chick, on the fixth day, has aiTumed a good deal of its mours have continued to increafe. 

 natural form. The exti-emities are fo much developed, that The 'white tends more towards the lower part and fmall 

 the divifions of thefe members are vifible. The wings have end of the egg : it continues to diminifh, and to become 

 their bent figure, and the legs are added to the thighs. more tenacious in its compofition. 



This period ishkewife marked by the firil appearance of The j'»/i is more fluid, more fpread out, and apparently- 

 mod of the important org.aus of the body, befides fome of a larger fize than before. Haller neverthelefs found it 

 changes in thofe already formed. at this date to poflefs its original weight. 



Haller ftates that he could difcern xht pericardium at this The i/oo(/-i'^/r which are diftnbutcd upon the membranes, 



time. and which were in the firll dap, as before meut^ncd, of a 



The two auricles are ftill more diRinft than at the end of brilliant fcarlet, are now in moll eggs found to be all of 



the preceding day. , the purple or venous colour. The uniformity of colour 



The auricular caiuil, which had before begun to retire makes it difScult to diftinguiih the arteries : ihey can in- 



tvltliin the flefli of the heart, is now not vifible. deed be only afcerta'ned by diuefting the membranes and 



The -venlricks are better formed, and broad at their purfuing the courfe of the velTels into the body of the 



bafe. chick. 



The lulb, ^ejlrait, and the leal of the aorta, which gave ' The amnios is ftill larger at this tlian any previous pe- 

 ovigin, in the early periods, to the princi;>al arterial trunk, riod; it prefervcs its kidney form, and its fluid is tranfparent, 

 are in general not apparent at the end of the fixth day. and cannot be rendered opaque, or -coagulated by vinegar or 

 This veffel is feen to come forth from the bafis of the heart fpirits. 

 itfeif. The -officula umhilicaUs goes on incrcafing in fize, and its 



"VIk pulmonary artery was fccn by Haller firft, about the veflels acquire ftrength. 

 middle of the fixth day. This veffel is lei's firm and opaque The chick has its members more dev^hiped. The feet are 

 than the aorta ; it arifes from the L-ftfide of liic bafis of the added to th.e poderior extremities, and even the tots can be 

 heart with a little bulb. It is ufu.^lly of lefs diameter than difcerned. O.Tification has commenced. Haller ftates that 

 the aorta. Almeft at its exit from the heart it divides into he prepared i[\(i femur and tibia at the 146th hour. The 

 two brandies. fa-o mandibles begin to form at the under part of the head. 



The branch on the left fide forms an arch, in order to leaving an aperture between them, which is to be th? 

 reach the lung cf that fide in pafiing behind the bronchia : mouth. _ 



it joins the aorta, towards the thud vertebra of the back. Haller at this period obferved the firft convolutions of 

 The branch of the pulmonary artery, which is inferted the fnufl infe/lines, which were fo extremely tender as not 

 into tl:e dorfal aorta, is longer than the di:c!ar arteriofus of to admit of being touched, although the reAum was appa- 

 quadrupeds, to which it is analogous. It fliould, how- rent two days before. 



ever, r.ceive the name of left, as birds have a fimilar ar- The fpL-en now firft makes its appearance. Haller fup- 

 ferial duft upoo the right fide. The left lung receives aeon- pofcd that he could likewife difcern the renal capfules and 

 Uderabk branch from the dudus while on its way to join the ow.ries. 



aorta. The intermediate of the three veffels which arife from tlifi 



Tiie other principal branch of the pulirionary artery heart to form the dorfal aorta, is, about this time, feen to- 

 paJjTes tranfverfely to the right fide, under the aorta ; to furniih the /oto carotids ; it is the fame velTel wliich be- 

 which it adheres. It furnillies the artery to the right lung, comes the defending au-ia, when the ftrutiure of the pul- 

 and defcends behind the bronchia, in order to open into monary artery is completed. Haller dates that the puifa- 

 the aorta, higher than the left branch already mentioned, tion of the aorta ceafes after the 154:!! hour. 

 The right arterial duft is much fmaller than the left. The eye of the foetus is confideiably developed at the- 



Thele arterial du<;ts arc two of the three branches, which prefent period. The three huniours exift : the r/.'rfouj iff 



3 extremely^ 



