INCUBATION. 



entirely the drop of blood it contains, and therefore it re- 

 tains almoft conftantly a degree of red colour. 



The wave of blood whicli proceeds from the auricle ap- 

 pears to vanifh, as one cannot fee its courfe into the ventricle 

 unlefs the heart be beheld obHquely from above : then the 

 blood will be feen to pafs along through the auricular canal, 

 which, as before obferved, is concealed by the ventricle. 



The contraftion of the ventricle fucceeds to that of the 

 auricle : it approximates its fides, and becomes narrower and 

 ftiorter. The ventricle frequently becomes quite pale during 

 its contraftion, owing to the entire expulfion of its contained 

 blood. 



After the contraftion of the ventricle follows that of the 

 bulb of the aorta, which pulfatcs as quickly and perfeftly as 

 the ventricle itfelf It empties itfelf completely at each 

 contraftion, and therefore appears quite pale at thefe 

 moments. 



The pulfatory movement terminates at the origin of the 

 three veifels which go to form the dorfal aorta, and this vef- 

 fel declines very much in bulk oppolite to the umbilicus ; 

 the branches which go to fupply the inferior parts of the 

 foetus being very infiguificant. 



The different membranes become more diflinft at the end of 

 the third day than they were before. At this period alfo 

 the firft appearance is feen of that remarkable membrane, 

 which is called by fome aHantois, and by others veficula urn- 

 bilicaltj. 



When a portion of the membrane is removed containing 

 the vafcular area and the chick, and watlied and fpread out 

 in clear water, it is found to contain four layers. The firfl 

 is thin, tranfparent, and iliining, and rcfembles that which 

 forms the yolk-bag before incubation. It very readily fe- 

 parates from the other layers. The fecond is alfo tranfpa- 

 rent, and ftill thinner than the firft, but is more adherent to 

 thofc underneath it. The third is the membrane in which 

 the blood-veffels are diftributed that form the vafcular area ; 

 and the fourth is the internal or fpongy membrane of the 

 yolk-bag. The third membrane feems to grow with the 

 vafcular area, by which it appears to be bounded : the inter- 

 nal membrane of the yolk-bag is at this period a little 

 fpongy or villous at every part, but much more fo under the 

 vafcular area than .it any oUier place. 



The chick inclofcd in the amnios is fituated between the 

 two firft and two la ft of thefe membranes. 



The vefcula uml/ilicalis, when firft difcovered, makes its 

 appearance as a fmail fpherical tranfpareut bag, which pro- 

 jects out of the opening into the abdominal cavity of the 

 chick, that is left by the reflection of the amnios inwards. 

 At this period, and for fome time afterwards, it is diftended 

 by a clear fluid. 



The end of the fourth day, or ninely-fx hours, the ap- 

 pearance of halcnes is in general no longer to be perceived. 



The vafcular area is fo much extended that it occupies 

 near the half of the fuperficies of the yolk : the outline lias 

 no remains of the heart fliape, but has affumed an oval 

 figure, the greateft diameter of which is in the tranfvcrfe 

 direction of tlie chick. 



The air-cell continues to enlarge at the great end of the 



. The external or liquid ivhite appears to be all confumed. 



The principal tvhlle is chiefly to be found in the lower 

 parts of the egg, <md when poured out is feen to adhere a 

 little to the internal layer of the membrane of the fhell at 

 the fmall end of the egg. 



The yoli-iag k ilill flatter in its figure than before, and 

 its contents are evidently more fluid than they were previous 

 to incubation. 



The chalatit continue to approach each other below the 

 poles of the yolk-bag. 



The amnios is confiderably increafed, contains more fluid, 

 and forms a deeper bed for the chick on the membrane of 

 the yolk-bag. 



The ivficula umbiHealis has made a rapid progrefs fince the 

 end of the third day. It now projcfts confiderably beyond 

 the fcEtus. It appears flatter, and lefs full of fluid than at 

 firft ; and at this period, by examination with a magnifying 

 glafs, a veflel may be feen to ramify upon it. 



Upon opening the yolk-bag, and waftiing off the yolk, the 

 fpongy coat is feen to be elevated in the courfe of the blood- 

 veflels of the vafcular area : this is the firft appearance of 

 the valvular ftrufture of the yolk-bag, of which more here- 

 after. 



The fatus is ftill more curved in its form, the prominence 

 corrcfponding to the bill being bent down upon the breait. 

 The rudiments of the upper extremities are apparent : they 

 are two white buds or Jhort projeftions. The lower limbs 

 are hkewife produced, but without any more form. The 

 cerebral veficles are more eminent ; and, when opened, their 

 contents are found, though ftill pellucid, to be mixed with 

 fome whitifh fibres, which form opaque clouds, and are the 

 commencement of the brain. 



Haller found the heart beginning to undergo fome changes 

 about this period. The auricle, when viewed in the ufual 

 manner on the right fide of the foetus, is feen to contain two 

 femicircles. The moft poftcrior of thefe, or the one neareft 

 the vertebrae, is the largeft, and terminates the auricle. The 

 anterior femicircle is fmaller, and divides the auricle as a kind 

 of feptum. This is the firlt appearance of the diftinftiou 

 of the auricles into two. That which is to be called the 

 left is the larger, and projefts over the right pofteriorly. 

 The latter, which is the fmaller, has a femilunar termination, 

 which is comprifed in the breadth of the left auricle. The 

 Tcna cava and the auricular canal are inferted in the interval 

 of the two auricles. 



The diftinftion of two auricles is likewife to be feen on 

 viewing the heart on the left fide. That which is to become 

 the left is oval, wider than the other, and paffes beyond the 

 auricular canal. 



At the end of the fourth day, or foon afterwards, the 

 two auricles become more feparate from the vena cava and 

 the auricular canal. They projetl over thefe veflels, and are 

 elevated from the two fides of the bafe of the heart into a 

 pointed top. The whole of the auricle, or the united refer- 

 vuir of tlie right and left auricles, appears then as a fac with 

 two pointed horns : each of thefe pointed tops has a fepa- 

 rate drop of blood. The border of both the auricles ap- 

 pears at this time notched. Mem. 2d fur la Formation du 

 Coeur dans le Poulet, &c. par M. de Haller, p. 70. 



The auricular canal, towards the end of tlu; fourth day, 

 paffes more in a direction from above downwards than for- 

 merly. 



According to Maitre Jan, the rudiments of the lungs are 

 vifible at this period ; but Haller afferts that they do not 

 appear before the 138th hour. The former author defcribea 

 at this time, in the neighbourhood of tin- heart, little ieddifl\ 

 clouds, apparently of a fpongy r.ature, winch iippear more or 

 lefs red, alternately, as the heart moves, and which remain of 

 an uniiorm colour when the heart has ceafcd to beat. Maitre 

 Jan admits that the pulmonary veflels cannot yet be difcern- 

 cd, but fuppofes from the above effeft of the heart's motion, 

 that tiicy do exlft, although they are not vifible. (Obfervat. 

 fur la Formation du Poulet, par Maitre Jan, p. 1 15 and 1 22.) 

 — Not having examined this point, we cannot decide between 

 thefe aOtlioritits, but we fliould judge it improbable that the 

 pulmonary 



