TNCTBATION OV RIKUS. 31 



iutelligeiit observer is at variiinee with the opinion so commonly entertained, that the 

 air was thus first supplied to the earliest-formed blood-vessels of the chick ; for it appears 

 not to come into use till the nineteenth day — which is within two days of the little bird 

 leaving the shell — when great changes are taking place, the large vessels of the 

 vesicle are to be set aside, and the function of respiration is to commence. From this 

 time, therefore, the large A-esscls shrink and shrivel, and, at the end of the twenty-first 

 day, the}- are detached from the body of the chick, and their remains are left in the shell 

 when the bird has quitted it. It is also worthy of remark, that the distension of the 

 lungs by air is the cause of the A'igour so necessary in lireaking the shell ; and it may be 

 added, as a curions fact, that the bag is not filled with atmospheric, but with a gas 

 specially provided. If the larger end of an egg be punctiu'ed with the smallest needle, 

 the embryo dies. 



On examining the contents of an egg between the third and fourth day, Mr. Towne 

 found on the lower part of the yolk, or that opposite the embrj'o, a white circular line, • 

 nearly the size of a lialf-penny. Continuing his inquiry, he observed the yolk-bag 

 within tliis circle became clouded, stretched, and wrinkled, and one egg among many, in 

 which the 5'olk-bag was ruptured within this cii-cle, but the yolk had not lost its form, 

 nor had the ^^•hite passed through the opening. On removing the portion of the thick 

 white ^^•hich adheres to the lower part of the yolk-bag from a goose-egg of fifteen days, 

 a great part of the contents ol' the bag inunediately escaped. He now observed a natural 

 opening in it, larger than a shilling, skirted by a vessel, which rendered the bomidary of 

 the opening so strong, that he coidd readily introduce his finger and withdraw it, which 

 he did repeatedly, without its giving way. 



It appears, therefore, that at the appointed time, the membranes of the yolk are rent 

 at that part op^iosite to the embrj'o, through which the thin white passes and imme- 

 diately mixes with the yolk, while the former sinks to the bottom, becomes more dense, 

 and actually plugs up the opening. It next becomes siu'romided by a vessel which 

 contracts, thus reducing the size of the opening ; and, as it does so, the thick white is 

 received within the yolk-bag, imtil, in the hen's egg, the white has nearly passed into 

 the yollc-bag, on the thirteenth or fourteenth day. The opening is now entirely closed, 

 and a very small portion of slimy matter is all that remains without. 



The manner in which the embryo is kept at the upper part of the egg, so that it may 

 be near the breast of the mother, has already been noticed ; but here another question 

 arises : for as there is, at the period just named, a great change of circumstances, how is the 

 chick f/irn found at the upper part of the egg ? !Most probablj^ the thick white, sinldng 

 to the bottom, by its own gravity, causes the embryo, which is exactly opposite, to rise 

 to the highest point. 



On the nineteenth day, when the extrusion is about to take place (fig. 24), the organ 

 which has, supplied whatever was necessary for the nutrition of the chick, is not com- 

 pletely exhausted. The residuum is now drawn into the body, which is then closed up, 

 the yolk entering the digestive tube. 



On the twentj'-first daj', the bird, disengaged from all its protecting membranes, which 

 have become dj-ied up ; ah-eady accustomed to pidmonary respiration, and with its first 

 necessities provided for by the yollc, is ready to leave the shell. But the walls it could 

 not break, were not the beak supplied with a film of carbonate of lime, so that it may be 

 used as a hammer, and the escape eflected. 



The writer has heard the sounds of the chick hammering at the inner shell with its 

 beak, but he has not been able to observe what Eeatnnur discovered, the little creature 

 at work. The result of the early eflbrts of the bird is a small crack, which is usually 

 nearer the larger than the smaller end of the egg. The membrane is rarelj- ruptured at 

 first, even when the hard part of the shell is detached ; and at this, that eminent 



