62 



OVUM. 



process, and in eggs that have been long kept, 

 the space has extended over the whole width 

 of the egg, and the quantity of gas contained 

 in this space is sufficient to cause the eggs to 

 float in water. The extent of the air-space 

 may be ascertained in some degree by the 

 greater or less feeling of coldness of the shell 

 of the egg near the obtuse end, when it is 

 applied to the skin of a delicate part, such 

 as the eyelid, in consequence of the heat being 

 less rapidly carried off by that part of the shell 

 within which the air-space has been formed, 

 than at others with which the albumen is in 

 contact. 



Dr. Paris found this air, amounting to about 

 half a cubic inch, to be nearly pure atmo- 

 spheric air, with a small quantity of carbonic 

 acid towards the end of the period of incuba- 

 tion. MM. Baudrimont and St. Ange find 

 it to contain in general more oxygen than 

 atmospheric air, and no carbonic acid ; whence 

 they conclude that the shell has a peculiar 

 power of passing outwards the carbonic acid 

 formed during incubation and taking in 

 oxygen.* The formation of the air-space is 

 manifestly a compensation for the loss of sub- 

 stance in whatever way occasioned, that may 

 take place from the egg. We shall have oc- 

 casion afterwards to consider in how far it 

 may be important in connection with the 

 phenomena of incubation. 



The specific gravity of the whole egg, when 

 newly laid, and before evaporation has taken 

 place, is generally as high as 1090, being 

 raised considerably above the common spe- 

 cific gravity of the fluids and soft parts of 

 animals by the superior density of the shell ; 

 but as the air space increases, the whole spe- 

 cific gravity will be lowered. 



The specific gravity of the albumen and 

 yolk differ in a considerable degree; that 

 of the yolk , though containing the largest 

 amount of solid matter, being lowest in con- 

 sequence of the large quantity of oily matter 

 belonging to it ; and thus when the albumen 

 becomes more fluid during incubation, the 

 yolk naturally rises towards its upper part, 

 or displaces some of the albumen which lay 

 above it in the newly laid egg. It is also 

 an interesting circumstance, that the specific 

 gravity of the lower and upper parts of the 

 yolk differs perceptibly ; that of the upper 

 part being reduced by the greater quantity 

 of oily matter contained in the cells situated 

 in the vicinity of the cicatricula. The up- 

 turning of the side of the yolk bearing 

 the cicatricula, which is familiarly known, has 

 long excited attention ; and several explana- 

 tions have been suggested of its cause ; and, 

 among others, the chalazae have been supposed 

 to balance the yolk in such a manner as to 

 secure this position. But Von Baer showed 

 that this view was erroneous, and that the 

 less specific gravity of the upper part, or of 

 that portion of the yolk in which the cicatri- 

 cula is placed, is the true cause of the phe- 



* Eecherches Anat. et Physiol. sur 1'CEuf des 

 Vertebre's, Mem. Couronn. ; published in Mem. des 

 Savans Etrangers de 1'Acad. Fran. 1850. 



nomenon. The measurements of the specific 

 gravity of different parts of the egg by Messrs. 

 Fig. 44. 



Position, form, and attachment of the chalazcc, yolk, 

 and cicatricula, as shown by sections of fowls' eqqs 

 boiled in different positions. 



A. Section of an egg, boiled on its side : B, with 

 the narrow end up: c, with the wide end up 

 These figures show the tendency of the lighter part 

 of the yolk, on the surface of which the cicatricula, 

 c, is situated, to be buoyed up and to expand in the 

 white, at the same time that the movements of the 

 yolk are to a certain extent limited by the attach- 

 ment of the chalazre ; a, air-space. 



