DEVELOPMENT OF THE CHICK. 209 



brane forms, extending itself as the membrane of the yolk extends, by 

 which means the yolk is rendered fit for passing through the duct into 

 the intestine, after the chick is hatched ; and it is even not coagulable 

 with heat, so that we may know when an egg has been sat upon, when 

 boiled, for the yolk remains a thin and watery fluid. 



As the chick grows, it presses down the middle of the yolk, first 

 making a deep indent in it ; and as it increases in length this indent is 

 increased into a groove, which becomes deeper ; and by the time the 

 chick is at its full growth, the yolk is almost divided into two portions, 

 between which lies the chick. 



When the chick is so far advanced as to have most of its parts begun 



to form, such as the extremities, which is about the hour 1 , 



then begins to form the third membrane, in form of a circumscribed 

 bag, which seems to come out from the belly near the anus, full of 

 water 2 . This, by increasing, spreads upon the chick, or over the above 

 membrane, and covers them, and as it increases, it covers the whole 

 albumen that remains ; and, as the slime diminishes, it becomes also a 

 covering for the yolk ; so the chick, albumen and yolk, are at last 

 enclosed by means of this bag ; but as it is a circumscribed bag in 

 itself, these parts are on the oiitside of its cavity ; but, by its forming 

 a circumscribed bag, in its double capacity it may be said to form two 

 circumscribed cavities ; and it is therefore to be understood that the 

 chick is only enclosed between this bag and the membrane of the yolk, 

 and is therefore not within its proper cavity, but upon its outside. 



This cavity, originally arising from the rectum, communicates with 

 it by a small duct, and probably is formed upon it, through which 

 passes the urine ; whence this cavity should be called ' allantois,' 

 although the membrane that forms the cavity has various uses ; it 

 absorbs the slime as it covers it, and therefore should be called placenta : 

 it comes in contact with the shell, and acts as lungs*. 



The urine in the chick is similar to that of the adidt, a white slimy 

 substance ; that which is in the allantois is firmer in texture, appearing 

 like strings of coagulated white of an egg, when thrown loose into hot 

 water. The water which it contained at first appears to be absorbed, 

 for none is found towards the last stages of incubation. 



* In animals that have [a urinary] bladder, this duct forms itself into that 

 cavity. In the crocodile the bladder opens into the gut, but in the quadruped the 

 urachus opens into or forms a passage for itself, called urethra. 



1 [In the embryo of the common fowl the extremities begin to bud about the 60th 

 hour.] 



2 [See Physiological Catalogue, 4to. vol. v. plate 71 /.] 



