OVUM. 



63 



- 1042'5 



- 1029'5 



Bauclrimont and St. Ange* are quite confir- 



matory of this view. They are as follows : 



Sp. gr. of the External albumen - 104T 



Internal albumen 



Whole yolk- 



Upper part of yolk - 1027' 



Lower part of yolk - lOSl'S-j- 



The chalazae, being of greater specific gra- 

 vity than even the inner layer of white, always 

 float lowest ; but, being attached to the yolk 

 near its poles, they hang down from these 

 points. All these circumstances may be illus- 

 trated very clearly by sections of eggs that have 

 been boiled in different fixed positions, as on 

 the side, on the large and small end ; in which 

 it will be found that, while the chalazae exer- 

 cise a certain control over the position of the 

 yolk, that portion of its surface containing- 

 the cicatricula rises higher and expands more 

 fully within the white than the opposite 

 portion, while the chalazae gravitate towards 

 the lower side. (See fg. 44.) 



Structure of the external parts of the egg. 

 The shell of the bird's egg is composed of a 

 delicate basis of organised animal matter im- 

 pregnated with the calcareous and earthy 

 particles, the arrangement of which approaches 

 to a crystalline appearance, but is probably of 

 a different nature. This substance is porous, 

 like concreted gypsum-plaster, and allows of 

 evaporation and the mutual diffusion of gases 

 through it in the same manner as that sub- 

 stance ; while, by its strength and rigidity, it 

 affords protection and support to the softer 

 parts of the egg during incubation. The 

 pores of the egg-shell may be easily stopped 

 by any greasy or oily matter, or by melted 

 wax or varnish ; and then all passage of 

 moisture or air through the shell being pre- 

 vented, the development of the embryo be- 

 comes impossible. Eggs that have been oiled 

 cannot, it is well known, be hatched ; but eggs 

 may be kept for a considerable time weeks, 

 or even for months by immersion in lime- 

 water, which impedes the evaporation and the 

 access of air, which might favour putrefaction, 

 while the natural condition of the contents is 

 thus preserved. 



The shell in most eggs is slightly dimpled 

 externally, with small depressions visible to 

 the naked eye ; but these are not the open- 

 ings of the pores through which evaporation 

 or exchange of gases takes place these being 

 much more minute and numerous but merely 

 the indication of depressions caused by the 

 largely villous structure of that part of the 

 oviduct (uterus) in which the calcareous 

 shell is deposited. 



On removing the earthy matter by means 

 of a dilute acid, the animal basis remains as a 



* Op. cit. 



t Dr. Wm. Aitken has, at my request, repeated 

 these experiments, and has obtained results in ac- 

 cordance with the above statement. He found the 

 unboiled yolk to float indifferently in any part of a 

 saline fluid of specific gravity 1035. By boiling, the 

 specific gravity was reduced to 1031, and in both 

 cases the side with the cicatricula floated upper- 

 most. The upper half, containing the cicatricula, 

 had a specific gravity of 1030 ; the lower half, 1032. 



slightly coherent, cellular, organised structure, 

 the form of the small compartments in which 

 corresponds with that of the calcareous par- 

 ticles of the shell (see fig. 45. c). The 



m- 



Fig. 45. 



a 



B 



Structure of the shell and shell-membrane in the 

 Fowl's egg. 



A. Lining membrane of the shell ; a, thick matted 

 or felty portion ; b, thin shred of the torn margin, 

 showing the peculiar fibrous tissue of which the 

 various layers are composed. 



B. Outermost layer of the same, which is incor- 

 porated with the shell; some of the angular cor- 

 puscles of the shell lying upon the fibrous substance 

 and firmly united with it. 



c. Small portion of the calcareous shell, which 

 has been steeped in dilute hydrochloric acid, show- 

 ing the remains of opaque calcareous substance in 

 the centre, some portions of it exhibiting a granular 

 aspect, and round the margins the animal basis or 

 matrix from which the calcareous matter has been 

 dissolved, presenting an irregular granular or almost 

 amorphous aspect. Here and there clear oval cells 

 are seen, as at a a. 



ternal surface is irregular and flocculent, and 

 adheres very closely to a different kind of 

 membrane which lines the shell. 



In those instances in which the shell of 

 eggs is coloured, the pigment substance, of 

 various hues, is generally deposited in cells, 

 which are strewed uniformly or in patches 

 over the external surface of the calcareous 

 shell. In some other instances, however, the 



