INCUBATION". 



- -loly abundant, upon whicli depends tlie great bulk of 

 ,.■3 of the chick about this time. The h-ris is fmall. 

 '. .: • irit h formed, but is very tranfparent. The trunks of 

 the op:ic n;rves and retina are %-ilible. The latter is tran- 

 fparent and plaited, and feems to go as far as the lens. The 

 radiaad black lines whicii form the corona cUinris are feen 

 through the retina, placed around the external part of the 

 Ions. 



The End of the 8th Day, or 1 92 Hours. — The vnfcular area 

 has incrcafed fo much by this timej that it very nearly 

 covers all the fuperticies of t!ie yolk-bag, ■except what is 

 in contail with the white. The branches of vefiels, how- 

 ever, begin to decline or difappear in fome parts, particii 

 larly towards the circumference, although the trunks are 

 larger than at any former period. 



The blood-vefll-ls belonging to the veficula umbilicalis are 

 conliderably enlarged and extended, and are now feen to 

 turn backwards, or be refleftcd upon themfelves in the di- 

 reftion of the chick. The reilecled portion of the vefTeis 

 of the veficula appears to be diiiribnted to the fup':rior half of 

 that bag which is now in contaft v.ith the external membrane. 

 This ir.enibra:ie, v.Iiich was in the firft perit;ds of incuba- 

 tion, as already mentioned, perfectly tranfparent, by its con- 

 Tieftion with the veficula becomes vafcular, and acquires 

 ftrength, and contributes to form a complete fac, which en- 

 velopes in the latter d.-,\s all the humours and the chick, as 

 will be hereafter defcribed. 



The veficula umbilicalis is found at the prefent period to 

 have a coniiderable extent, according to our obl'ervations 

 much greater than has been reprefented by anatomiRs in 

 general, who do not feem to have been acquainted with 

 the ufes of this bag in the later ftages of incubation. 



The chlcl is a very palo pink coloiu-, or white flightly 

 tinged v/iih red, in confequcncc of the vafcularity of the 

 furiace p.^eparatory to the formation of the fliin and feathers. 



When the head of the chick is viewed pofteriorly, it pre- 

 fcnts the appearances of four eminences, which are an opaque 

 white, and are joined to each other by tranfparent mem- 

 branes. Maitre Jan confiders them as the rudiments of the 

 bones of the craninm in a cartihginous Uate, and the tran- 

 fparent parts as correfponding to tlie futures and fontanel. 

 H.iUcr fpeaks of thefe eminences as four lobes of the brain. 

 We conceive the account of Maitre Jan to be more corrcdl. 



The head being opened at this period by Maitre Jan, he 

 ftates that he diitinguifiied ihe membranes, xhe Jinufes, and the 

 v^e's of the brain, and that the fubllance of the cerebrum 

 and cerd'ellum was well formed, the convolutions even bein^r 

 apparent. Tiiis obfervation, according, to our experience, 

 is premature. 



Tiie (IrucViire of the eye is more perfect tlian before. 

 The ci!i:iry •z.one, according to Halicr, is as perfcA at the 

 prefent period as at any time afterwards. Its defcription, 

 h jwever, is eafier fome days later. 



The bill of the chick is cartilaginous. The animal oc- 

 cafionally opens it in the water of the amnios as if attempt- 

 ing to fivalio-.v. 



The broail of the fcctus, which was covered by a thin 

 membrane, tiiat in the firll days was fo tranfparent, as to 

 give ri;e to the miilake of the heart being outfide the 

 chel^, hajat the prefent time fome fle(h upon it. About 

 this period alfo the rudiments of i\\<: Jlernum and of the 

 rilit may be difcovered. 



The gal'-lifjMr makes its firft appearance towards the 

 end of the eiglith day, at which period, and for a few days 

 aft'Twards, it is very fniali and without colour. 



.;'. ,.it this lime the refleftions of the inner tunic of the 



yolk-bag, wliich we have defcribed at the end of the fifth 

 day,-as putting on the appearance of yellow vefiels, acquire 

 more decidedly the form of valves. They lUU preferve the 

 fame fituation and courfe, coinmenciiig under the bed of 

 the fcctus, pnd extending outwards in a radiated manner to- 

 wards tlie circumference of the vafcular area, where they 

 termin.'.:e. Tliey are made of a number of ]>laits or folds 

 which have a tranfverfe direction, and whicli are more 

 prominent, or have deeper fojds in proportion as the val- 

 vu'ar li'.ses approach the circumference of the circle. Thev 

 form merely waves where they commence, and at the prefent 

 period they are on ev.ry part of the yolk-bag much lefs 

 marked and ilriking tlian they become towards the end of 

 intubation. 



The End of the gth Day, or 2 1 6 //(7ur/.— Maitre Jan mer- 

 tions the appearance of fome new parts at this period which 

 we have not met .with fo early. He defcribes the iidnies as 

 fecreting a miiky fluid, wl-.ich he faw diftending the ureters. 

 He diicovered about this time the crop and the trach a 

 aiid was not only able to diilinguilh th-i principal divifions 

 of the brain, but Hates that he could difcern the ventricles 

 with the eminences they contained, and even the plexus 

 choroides. He likewife faw upon the bafis of the brain 

 the greater number of the nerves that arife there, more par- 

 ticularly the optic and auditory nerves, and found the fpinal 

 marro'U) to be fplit or corapoled of two parts at its orio-in. 

 He difiected the nerves of the fpinal marro.v, which, al- 

 though very fmall, lie flates to be much more firm than 

 tho(e which take their origin from the brain. Obferv. fur 

 la Formation de Poulet, p. 1S9. 



The furface of the foetus exhibits now a number of pores ; 

 thefe are principally fituated upon the thighs, the back, the 

 neck, and the wings, and are the apertures of the cells in 

 which \\\(i fiiilhsrs are formed. 



The eyes are larger than the veficles of the brain ; their 

 form is not fpherical, but flattened upon the anterior and 

 poflerior fides. 



Tiie iris is flill thin and tranfparent ; but there are ffeen 

 throajh it and the tranfparent cornea five or fix white points 

 which- form a circle. When the eye is opened carefully 

 witli a lancet, thefe points are found to be elongated, and to 

 form httle lines, or ciliary fibres, which at this period are 

 white. 



Maitre Jan ftates the eyelids to be formed at this time, 

 but to be fo tranfparent as to be iuvifible, unlefs after pre- 

 paration by a ftrong acid. 



The htejlines are now fo firm, that they can be examined 

 without danger of breaking them. 



The yJk-bag, according to Maitre .Tan, begins to form 

 two lobes, a larger and a I'maller ; but we h.ave not obferved 

 this change to take place before the nth or i2lh day. 



Maitre Jan made the e;:periment of boiling an egg at 

 this period, after having fixed it upon a plate of lead whh 

 its" fmall end downwards. \\'hen defpoiled of the fliell, and 

 the chick renioved, the deprefiiou in which the amnios lay 

 v/as fully expofed, and found to be very coniiderable. He 

 then made a feftion of the egg in the vertical direction, di- 

 viding the depi-eflion of tiie yolk through the middle, when 

 thev; appeared a round fpace about the eighth of an inch iij 

 diameter, filled witii a whitilh curdy matter, and another 

 fpace around this of a larger extent, containing a" firmer 

 matter than the firll, and of a paler yellow than the rcll of 

 the yolk. Maitre Jnn, very erroneoully in our opinion, fup- 

 pof-d the white ciu-dy fubllance to be tlic inteftines of the 

 foftal chick. Thefe do not projeft from the abdomen in 

 fuffi .lent quantity to account for the appearance, admitting 

 that they had been leparatod from the chick, and Lad ad- 

 i) i here J 



