INCUBATION. 



intclliniform tubes upon the edges of the valves mofl beauti- 

 fully curled and folded upon themfelves. He failed to fee the 

 aperture of thefe tubes. By maceration in water they were 

 detached from the valves, and fwam in the water. The 

 valves theinlelves, by a longer maceration, were alfo fepa- 

 rated. 



The chid inereafes regularly in fize, though not fo rapidly 

 as during the early part of incubation. Haller obferved tlie 

 feathers upon the eye-lids at this period, and that the J]>leen 

 had acquired the figure it preferves through life. 



The curiiy, or cca^iilated fiib/lance is now found in the cefo- 

 phagus and the crop, as well as in the ttomach and inteftincs. 

 Harvey found this fubllance principally in the ftomach : this 

 appears to be the cafe, and when met with in other places, 

 it would feem to have proceeded from the ftomach to 

 them. 



The end of 16 days, or 384 hours, the fpace between 

 the two layers of the membrane lining the Ihell, for contain- 

 ing air, is much increafed fince the periad at which we have 

 lall mentioned it. About one-iixth of the whole cavity of 

 the egg is occupied by air. 



This circumllance depends upon the diminution of the 

 fubftance of the white, which is at prefent reduced to a very 

 fmall iize. 



The texture of the white continues to become firmer. 



Upon removing the chorion, which now completely en- 

 clofes the white, that humour is found to poilefs a thin 

 tunic, which is proper to it. This membrane is evidently of 

 a late growth, and appears to be formed by the vefFels of the 

 yolk-bag extending beyond the feptum, or that portion of 

 the bag which is bounded by the vafcular circle, and wliich 

 is in contaft with the white. 



The fluid of the amnios is fomewhat'diminifhcd. 



The yolk-bag in the egg of the hen is generally formed into 

 three lobules, although many authors fpeak only of two 

 lobes at this time ; one, which is tlie moll; dillinct, but the 

 fmalleft, lies upon the fide of the body of the chick, between 

 the head and the thigh : the two others are placed at the two 

 ends of the chick. 



Maitre Jan defcribes the yolk-bag as being formed only 

 into two lobes, but ready to enter the abdomen, of which 

 he gives a figure at this period, and of a fiffure that afcends 

 from the umbilicus, between the two redi mufcles of the 

 belly of the chick. He likewife defcribes the duftus vitcllo- 

 inteftinalis for the firll time, which, with the yolk-bag, he 

 fucceeded in inflating from the crop of the chick ; and he lays 

 this part is not to be demonllrated at an earlier period than 

 the end of the i6ih day, on account of its extreme tender- 

 nefs ; but, as already mentioned, we have found the duft on 

 the 13th day. 



Haller found the inteftines before this period to be irritable. 

 When ftimulated, they contrafted into knots. 



The blobd-veflels of the yolk-bagappear fewer and fmallcr, 

 owing, in part, to their being fo much more involved in the 

 valvular coat than they were in the middle periods of incu- 

 bation. The branches of the blood-velfels of the yolk-bag 

 are larger than the trunks, and the veins are llill vaftly 

 greater than the arteries. Haller ftates that the vein going 

 upon one of the valves, is a larger velfel than the trunk, from 

 whence all the arteries of the yolk-bag take their origin. 



The End of 17 Days, or 408 Hours — Haller admits the 

 appearance of tlu-ee lobes of the yolk at this date, which we 

 have fcen before. 



The arteries of the chorion are extremely large ; we 

 have fucceeded in introducing a ; ipe into them, and injeft- 

 ing them with coloured fize, which makes a beaut ii'ul prepa- 

 lation. Haller, in his obfervation of this date, found both 



the arteries and veins of the chorion to be a purple colour, 

 and the veins more violet than the arteries. 



It now appears that there are two feries of vctTels belong- 

 ing to the yolk-bag. The branches, which arc fpread upon 

 the external membrane of the bag, appear to be the fane 

 whicli formed the vafcular area in'the early periods of incr- 

 bation. The veflels of the internal coat are dillributcd :-) 

 the valves. Both feries contribute to the vafcular cirtl ■, 

 whicli, as we have before ftated, is the remains of the v. 1 1 

 that formed the contour of the vafcular area. Maitre J~n 

 fays there are fcveral layers of the membrane which contaiis 

 the yolk, but it is probable that he miftook for one that 

 membrane which inclofes the yolk-bag, tiie inteftines, and .1 

 part of the chick. 



Coiter has found the yoli-bag within the belly of the chick 

 at the 17th day ; but the ingurgitation of the yolk is a later 

 event, according to the obfervation of other writers. 



There is now a circular arrangement of mufculnr fibr-^i 

 around the opening into the belly of the chick at the umbi- 

 licus. This fphintler was not apparent before, and, as wi> 

 flrall foon find, is a very efiential llruclure in the lall periods 

 of incubation. 



If the yoU-bag be opened, and the valvular membranr; 

 cleaned by frequent ablution and floated in clear water, ihc- 

 fides of the valves will be found to be perfo.atcd by a great 

 number of foramina, fo as to give the valves the appearance 

 of lace. The inteftiniform tubes alfo upon the edges of the 

 valves will be partially detached ; the portions that remain 

 refemble pieces of cy!indric tubes. 



Many of the blood-velTels upon the external furfacc of the 

 yolk-bag contain apparently but little blood, and have a 

 grecniih colour, as if conveying a yellow or green fluid. 



The End of the 1 8//6 Day, or 4J 2 Hours The two layers 



of tlie external membrane, which immediately fines the fhell, 

 are feparate for about one-third of their extent, and can with 

 little pains be detached from each other until near the fmall 

 end of the egg. The quantity of air contained between 

 thefe layers, although much increafed, is lefs than might be 

 fuppofed, as the layers are not far afunder, except at the 

 great end of the egg. 



The '•jjaltr of the amnios is a good deal diminiflicd, and its 

 membrane is apphcd in folds to the furface of tlic chick. 



The chorion has attained a very confidcrablc ftrength, ad- 

 mitting of examination and diffccftion, without being prc- 

 vioufly hardened by fpirits or vinegar. When this mem- 

 brane is laid open, there is fome tough lymphy fubftance 

 found under it in layers or llreaks, which has been miftakcn 

 for a membrane by fome anatomifts. There is generally mixed 

 with this fubftance a ceitain quantity of calcareous matter. 

 It has, therefore, been fuppofed to be a kind of excrement, or 

 the fecretion of the kidnics. Thefe white flakes are what 

 have been called^y?)-/',? retictdata urinir by Malpighi, and were 

 defcribed by him as being found in the allantois. 



When the chorion is opened and turned down off the con- 

 tents of the egg, it appears to be a diftinft bag, and only 

 conneifted to tlie other parts by refleftionsof fine membrane. 

 Tiiefe are chiefly in the courle of the trunks of its blood- 

 veflt'ls, and at the junction of the yolk and white. 



The yoli-tag is now often found gathered into more lobules 

 than three, fome of which have entered the abdomen of the 

 chick, through the aperture at the umbilicus. The forming of 

 the yolk-bag into lobes and lobnles, appears to be o»ing to 

 the contraction of the membrane that contains it, and the in- 

 teftines of the chick ; for, when the yolk-bag is taken out of 

 this membrane, it fpreads out into nearly its original figure, 

 except that there is a marked deprefTion where the white is 

 joined with it, and that the neck of the bag is fomev\ hat elon. 



ffatcd 



