INCUBATION. 



hered to the yolk during the boilincr of tlie egg. This 

 appearance, we believe, arifes from the fame change of orga- 

 nization in the yolk being continued, which in the early 

 periods of incubation produced the whitifh appearance under 

 and around the bed of the chick. The firm, pale yellow 

 texture of the yolk which encompafTes that juft mentioned, 

 Maitre Jan accounts for by fuppofing that the fluid parts 

 of the yolk in the neighbourhood of the chick had pafTed 

 into the amnios. There can be no doubt that different parts 

 of the fubllance of the yolk are differently conftituted, 

 which difference is prefcrvcd notwithflanding the motions 

 that eggs may experience ; a fadt that cannot perhaps be 

 fully explained, but which proves that animal fluids have 

 their peculiar organization, which they can maintain as well 

 as the folids. When tlie fluids change this compofilion, it 

 is often by fome vital aftion in themfelves, inllead of a fer- 

 mentative proccfs, or the mechanical operation of the folids 

 upon them, as is commonly fuppofed. 



In making the experiment of boiling eggs at this period, 

 Maitre Jan did not find that the amnios was coagulated, al- 

 though Haller Rated that the liquid of the amnios is coagu- 

 lable at the 198th hour. 



The valvular ^niSurc upon the internal furface of the yalk- 

 bag is more prominent, and when minutely examined, it is 

 found to receive branches of t)ie blood-veffcls which are 

 given off at right angles with the plane of the furface of 

 the bag, and having penetrated towards the edge of each 

 valve, there unite and form a fingle vefTel, which runs 

 within that edge. This ftrufture is better feen a few days 

 afterwards. 



The End of the 10th Day, or 240 Hours. — The I'ljicula 

 vmb'tHcaHs has now a very coniiderable extent, occupying the 

 greatell part of the furface of the vafcular area, v.hich laft is 

 fpread over almoft the whole of the yolk-bag. The portion 

 of the bag that is applied to the white, is not at this date 

 the |th of the whole. The chalazce, therefore, are much 

 nearer to each other than when we laft mentioned them. 

 The while is confined almoft entirely to the lower part of 

 the egg, and its mafs continues daily to diminifh, while, on 

 the contrary, the bulk and quantity of the yolk appears to 

 be increafed. 



The exterr.al memlrane is firm and flefliy where it is 

 ftrengthened by the velicula umbilicalis. Some of its vef- 

 fels are feen to extend to the white, upon which a fine 

 pellicle may be obferved to grow, and which is the firll ap- 

 pearance of a membrane to this humour, notwithllandiiig 

 fome authors fpeak of the membrana albuminis as an original 

 ftrufture, and others, amongll whom is Biumenbach, dc- 

 fcribe the membrane wliich lines the fliell as belonging to tlie 

 white. 



The amnios is larger, its membrane more firm, and the 

 fluid lefs clear and limpid than before. It has contraded 

 fome adhefions with tie vcficula umbilic;.lis, and by that 

 means, with the external membrane. Thefe adhelions are 

 particularly in the courfe of the blood-vcflcls, that are 

 dillributed to the veficula and external membrane. One of 

 thefe veiiels is always feen to pafs acrofs the back of the 

 chick, at which place it is involved in a rcfleftion of very 

 fine membrane, which is connected with that of the amnios. 



The chick is very much curved at this period : its heak is 

 ufuiliy doubled down between its feet. The feathers are 

 apriarent. 



The external form of the head bears a greater refembbnce 

 to that of the full-grown bird, except that the eyes are P. ill 

 enormouny large. 



The eyelids are evidently formed, and are occafiosaUy 



moved by the chick. The membrana niSitaiu is likewife to 

 be feen. 



Haller prepared at this period the ciliary zone and radiated 

 appearance called the crozun, with the membrane on whicii 

 the latter is fuftained, although he ilates that thefe parts are 

 formed on the 8th day, or before it. 



He fuys he has feen on the 7th day the corona ciliaris, or 

 corpus ciiiare of Morgagni. At this epoch the retina, which 

 is in thick folds, appears to go as far forwards as the cryf- 

 talhne lens, behind the corOna ciliaris, with which it is co- 

 vered, and which is dimly feen acrofs it. He believes, not- 

 withitanding, that the retina does not proceed to the lens ; 

 that the appearance of its doing fo arifes from its thicknefs 

 and folds leaving fo fmall a fpace between it and the lens, 

 and that the membrane wliich fupports the ciliary zone and 

 crown, and is attached to the cryllalline, is different from 

 the retina. 



The corona ciliaris is a row of black lines arranged alter- 

 nately, long and fhort, in fome degree parallel, but con- 

 verging as ravs proceeding from the external circumference 

 of the uvea to the cryflalline lens. The ciliary crown is- 

 not formed entirely in tlie fame manner in quadrupeds and 

 birds. In the former it appears, when examined by a micro- 

 fcope, to be compofed of a plicated membrane, upon which 

 fome rami.fied veffels advance towards the cryllalline. la 

 birds there are only fome ferpentine lines folded upon them- 

 felves, which are very numerous in the contour of the ciliary 

 crown, and of wliich many unite to form fingle ones : they 

 are joined toget'ier by thefe branches, aad arc covered with 

 a black pigment. 



The figure of the ciliary zone and crown is not a perfeft 

 circle, but it is broader on one fide than the other. 



The greater number of anatomilfs have defciibed the co- 

 rona ciliaris as relHng immediately upon the membrane of 

 the vitreous hurhour. Others have beheved that the retinal 

 extends to the cryftalline, and that the corona is fupported 

 upon it. Since Zinn's pubhcation u;)on the anatomy of the 

 eye, the ftrufture of the ciliary body in man and quadrupeds 

 has been well underftood ; but in birds the lamina, which 

 ferves for the bafe of the corona ciharis, does not come from 

 the membrane of the vitreous humour. 



The membrane, which is placed under the corona ciliaris in 

 birds, is to be feen from the Sth day. It is radiated, opaque, 

 and a little alh-coloured ; more thin than the retina, and 

 thicker than the membrane of the vitreous humour, it is 

 of the fame dimenfions as the corona ciliaris, and in a degree 

 attached to llie corona, aad much more firmly united in the 

 latter days of incubation ; and after the exllufion of the 

 chick, it adheres by its psltericr extremity to the great 

 circle of the retina, and by the other to the cryllalline 

 lens. 



The retina terminates by a diftinftly formed circle, which 

 is a little llrengthened at the circumference of the corona 

 ciliaris, and at that of the zone. The plane of the retina is 

 continued with that of the corona ; the zone is placed more 

 polleriorly than the retina. Its external part, which reils 

 upon the vitreous humour, is nearly horizontal, and its in- 

 ternal part becomes llraight again in order to be attached to 

 the cryftalline lens. It does not adhere to the membrane of 

 the vitreous humour when the chick is but little advanced. 

 It remains often placed upon that membrane without being 

 attached to it, and wlien the chick is more formed, or after 

 incubation is finifticd, it quits the membrane of the vitreous 

 humour, in order to remain connefted to the corona ciliaris, 

 with which it becomes infeparably united in the ond. 



The circumference of the cihary zone, in the latter days 

 of incubation, can be feparated froia the membrane of the 



vitreous 



