DEVELOPMENT OV THE CHICK. 215 



The parietes of the abdomen are the latest in being formed, and when 

 that is effected the animal is completed ; but this is much later in some 

 of the oviparous animals than in the qiiadruped, and the lateness of 

 forming this part is owing to the yolk's being taken into the cavity of 

 the belly at or near hatching ; and to effect this purpose we find that 

 from the circumference all round the opening into the belly arises a 

 muscular expansion which enters the yolk (besides its proper mem- 

 brane], surrounding the whole, which by its contraction draws the yolk 

 towards the opening, and then by its contraction that part of the yolk 

 which is close upon the mouth of the opening is shoved into the belly; 

 and by this action some of the yolk (which is become fluid) is squeezed 

 into the intestine, which by regurgitation in that canal is carried up 

 into the stomach, and is there first coagulated to be afterwards digested. 



As birds have air-bags within the abdomen, I find that at a certain 

 period of growth of the chick they begin to form. They begin at the 

 lower point of the lungs like a small hydatid, and extend further and 

 further into the abdomen, before and on the outside of the kidneys : 

 they are at first full of a fluid ; as they extend, they are, as it were, 

 squeezed among the intestines, so as to take on the shape of the intes- 

 tines of those parts, and at last filling the whole abdomen among them. 

 Soon after others are forming, or other openings communicating with 

 this, and the lungs are also beginning to attach themselves so as to form 

 a communication with other parts, as the ribs, sternum, vertebras, &c, 



The lungs are, at first, detached bodies, as in the quadruped, but 

 when arrived about the third week (in the goose) they begin to be 

 attached to the ribs, but not so early to the diaphragm. 



Among the latest formed parts of the chick are the eyelids. "When 

 gone through one half of their period of incubation, the whole anterior 

 surface of the eye is exposed, and the termination of the common 

 integuments is perfectly round, as in fig. [12. plate 76, torn. cit.~] But 

 in a day or two more it begins to form itself into an oblong opening, as 

 in fig. [16. plate 76, torn. tit.], which becomes narrower, as in fig. 

 [5. plate 75, torn, tit.'], and then the increase of lid becomes more remark- 

 able in the lower lid, becoming first almost straight, as in fig. [17. plate 

 76, torn. tit.], which afterwards becomes rounded on its edge, almost 

 covering the whole of the lower pari of the eye, as in fig. [18. plate 

 76, torn. cit.~], and about a day or two before they are hatched the lower 

 lid has spread upwards so much as almost to cover the whole eye, as in 

 fig. [plate 78, torn, tit.'] 



The membrana nictitans begins earlier to form than the eyelids, for 

 in fig. [16. plate 76, torn, tit.] it is seen at a, and its increase may be 

 observed in all the other eyes at letter a. 



