INCUBATION. 



Yitreous humour without leaving any (hrcds. It is dillin- 

 cuidicd from the retina by its linencfs and being feparable 

 from tlie latter without being torn ; but it is continued with 

 the pofterior and inferior part of tliat membrane. It differs 

 ttill more from the membrane of tlie vitreous humour ; it 

 is thicker, it has more brown and lofs blue in its colour. 

 Acids, when applied to it, atfeft its colour more than they 

 do that of the vitreous membrane. It is fituated anteriorly 

 upon that membrane, in another plane, and evidently fui^- 

 tained by it, and confequently ought not to be confounded 

 with it. 



The canal of Petit does not exift. The membrane of the 

 vitreous humour proceeds, without (lopping behind tlie cihary 

 zone to be attached to the cryitalline lens, that it fuftains, 

 and which remains in its place even when the corona ciliaris 

 has been i ailed. 



The cihary zone is eafy to prepare even in the chick but 

 little advanced. One method is to cut off the cornea and iris ; 

 and having wafhed away the black pigment with which the 

 corona ciharis is covered, it will appear between the intervals 

 ©f the ciliary rays ; and if iome of thofe rays be elevated, it 

 will be feen in its proper place. It is practicable to make 

 the leas come forth from its deprelTion, and to elevate it with 

 the corona ciliaris, « ith which it is connected ; and fometimes 

 it may be poffib'e to leave the zone relHng upon the mem- 

 brane of the vi'.reous humour. 



An eafier mode of preparation is to cut the fclerotic c:5at 

 through the middle, and to plunge the anterior half of the 

 eye into vinegar. After fome hours immerfion in that fluid, 

 if the back of a fine fcalpel beinfiiiuated between tlie retina 

 and the choroid coat, and the vitreous and cryftalline humours 

 be removed, one fometimes fucceeds in obtaining the ciliary 

 zone alone, the corona remaining attached to the choroid. It 

 is rare to have the zone entire, but a conliderable part of it 

 is commonly preferved, ai.d particularly the whole of the ex- 

 ternal circle. When the experiment fails, it is the interior 

 circle which remains attached to the corona. 



In the above account of the ciliary zone and crown, we 

 have anticip.ited many facts that are obfervable during the 

 laft days of incubation, and even after that procefs terminates, 

 in order to render the hillory of thofe parts more intelligible. 

 We have alfo been induced to borrow the preceding deicrip- 

 tion from Haller, who made the llrudure of the ciliary zone 

 and crown in the cluck a particular fubject of inveiligation, 

 as we have not much attended to the progrefi; of the eye dur- 

 ing incubation. See Mem. fur la Format, du Cosur dans le 

 Poulet, furTGEil, &.c. Obferv. i8o, i8i, 1S2, 191, 192, 

 153, 215, 216, 226, 228, 231, 239, 240, 247, 249, 25^, 

 254, 256, 261, 262, Sec. and feCl. xii. jSlem. i^ecoud par I\I. 

 Haller. The f mall tubercle, from whence the aorta and pul- 

 monary arteries arife, and which appears to have been a 

 remnant of the bulb of the aorta, entirely difippears abt)ut 

 the prefent period. 



The right ventricle of the heart is dill about the one-fourth 

 of Its length Ihorter than the left. 



Thofe bones of the chick, according to Hallcr's obfcrva- 

 tions, which are nov.- formed, are merely in a cartilaginous ftate ; 

 even the cranium, which was apparent fome time before the 

 prefent. 



The umbilical opening, through which the urachus and the 

 blood-veffels communicate with the interior oi the body of 

 the chick, forms a projection, or a fort of tube, which in 

 a few days becomes more ilriking. Some of the convolutions 

 of the inte!iinesmay be feen in this opening, but the grcateil 

 part of the intellines is contained in the body of tiie foetus. 



The end of the nth day, or 264 h'yurs, the external mem- 

 brane goes on llrcngihenmg, by its connexion witli the vejii^u- 



la umbilicalis. This lafl is now fo extcnfively u.'iited to the 

 external membrane and the amnios, that it is only by great 

 care, and after being macerated in fpirits, that it can be li- 

 berated from its connections, and cxpofed as a dHlindt bag, 

 pendent from the umbilicus. AVhen thus feparattd by dif- 

 feftion, it is feen to be a large fac, compofed of a thick Iflefhy 

 membrane and hardly containing any fluid. The Urength 

 of the bag appears to depend upon a rcfledlion of fome of the 

 other membranes covering and being infeparably adherent to 

 it, for at the umbihcus tlie velicula is thin. Haller traced the 

 urachus from the velicula to the reftum after tlie prefent 

 period, and even until the lafl day of incubation ; but in. our 

 obiervations it has appeared that the connection of the vefi- 

 cula with the chick, except by means of the blood-vefTels, 

 after this time, is verv obfcure. 



The whole of the body of the chick is now feen to be 

 covered whh feathers. 



Haller at this period firft difcovertd the tongue. 



The fubftance of the brain is Hill found to be a very foft 



The liver, in moft inftance?, has a yellow colour, but in 

 fome the red predominates. 



L'ik ii evidently fccreted : the gall-bag is green, and a 

 grccnifh or yellow fluid is commonly found in the ilomach. 

 The inteftines hkewife appear to contain bile, or at lealls a 

 bitter fluid. 



The retial capfules are evident ; they have, accordinc: to 

 Haller, the form of an S. 



The tube or fheath which projects at the umbilicus is bet- 

 ter formed. 



T\\e blood-vejfels, which come out at this place, are eafily 

 traced from different fources. Thofe which are diltributed to 

 the external membrane and veficula come from the loins, and 

 thofe belonging to the yolk-bag are from the abdomen. 



The end of the 12th day, or 2S8 hours, the chick has more 

 of the figure of the adult bird than at any preceding period. 

 The extremities are larger, and the head fmaller, in proportion 

 to the reft of the body. The bill is more pointed, and of a 

 harder fubllance. 



Thejlomach has gained greater ilrength ; it contains a 

 ■white coagulated or curd-like fubftance. 



There is bile found in all the inteftines, whicli is very 

 bitter. 



On the outfide of the umbilical fheath there are fome 

 folds of the inteftines, with one of which the yolk-bag is feen 

 to be connected by a (hort procefs, which is evidently the 

 continuation of tiie alimentary canal into the yolk-bag. 

 This part has commonly, and as we fliall hereafter' fee very 

 defervedly, received the name of duSus vltello-inte/linalis. Al- 

 though we have not obferved the duft before the prefent 

 date, there is every reafon to fuppofe that it is coeval witlu 

 the inteftines. 



iVIalpighi ftates that he faw the Jlomnch of the chick 

 on the outfide of the umbilical ftieath : but this would fcem 

 to be an error. The ftomach, it is true, is fituated low down 

 in the belly, but, as far as our obfervation goes, it never is 

 protruded from that cavity. 



The ftrufture of \.\\(i eye is the fame that is defcribed on 

 the nth day. It is proper to remark, that at no period 

 is there to be found in the chick a membrana pupillaris. 



The yoli-bag, according to our obfervations, begins in 

 general to alter it s figure. Before this date it was an irregularly 

 round mafs, containing two deprelTions ; one for accommo- 

 dating the chick and amnios at the upper fide, and another 

 at the lower part of the yolk, in which the white is partially 

 lodged. The latter is a regular dcprt (Tion, and before- this 

 period contracts in breadth, although it becomes fome what 



greater. 



