290 EMBEYOLOGY. 



STETJCTTJEE OF THE EGG AS JUST LAID. 



[ 4 SO. The egg of the common fowl is surrounded exter- 

 nally with a hard calcareous shell (fig. 287, #), consisting 

 almost wholly of carbonate of lime. It is, indeed, without 

 obvious pores, but is nevertheless permeable to air : some part 

 of its watery constituent escapes during the process of hatch- 

 ing, and eggs that are covered with a coat of varnish die. 

 Internally the shell is full of pits or depressions, in which 

 small warty or shaggy processes of the lining membrane of the 

 shell (the mernbrana testae} are implanted (fig. 287, c, c). 

 This membrane consists of two laminse, the outer of which is 

 made rough and uneven by the processes just mentioned ; 

 the inner, which is turned towards the white, is smooth and 

 polished. The two laminae separate at the blunt end of the 

 egg (fig. 287, d, d), so that here they are most easily demon- 

 strated, and contain the air-space, or air-chamber (folliculus 

 aeris) between them, which first appears shortly after the egg 

 is laid, and is very much enlarged by keeping and the heat of 

 incubation. The membrane of the shell is formed of a com- 

 pact fibrous tissue, and shows the chemical properties of 

 coagulated albumen. Betwixt the membrane of the shell and 

 the yolk is interposed the white (cilbumen ovi), the outer 

 stratum of which (fig. 287, between c and e) is extremely 

 watery and fluent, and consequently readily drained off 

 when the shell is pierced ; the inner layer, again, or that 

 which lies nearer the yolk, is more viscid and thicker 

 (fig. 287, between e and /), clings more closely to the 

 yolk, especially by its inmost stratum, which immediately 

 surrounds that part and the chalazse (fig. 287, /, /). The 

 white of an egg shows alkaline reaction, and contains a^bu- 



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Fig. 318. One of the chalazae of the jackdaw's egg pulled straight. 

 The way in which the twisted fibres of the part diverge into a funnel- 

 shaped expansion as they approach the volk, and so form the innermost 

 stratum of the albumen, is displayed. 



