xiv ELEMENTS 01- VERTEBRATE i.Mi;kY< >I.<)GY 457 



towards the external opening in IN passage down the o\ Mong 



the long axis of the mass of albumen -from the \itdlinr m< mhrane to 

 each pole there passes a denser less transparent strand <! albumen 

 known as the ehaten (Fig. 191, &). Thechalaza iirmrl 



portions of tin- albumen which preceded and followed alter t 

 its ]. ion- the oviduct. During this pass.c 4 rotates 



slowly on itself and the inner ends of the- chala/ar IM in- aiia In -I to the 

 vitelline membrane these structures are -i\cn a < h.. t in 



opposite directions. The chalazae serve to keep the e-- in the mid 

 the albumen while allowing it to rotate freely about the loi ! tin- 



mass of albumen. As the germinal disc is in a position which is practi. .dl\ 

 equatorial in relation to this axis, or in other words it is as far as possible 

 from this axis, and as further 



the cytoplasm forming it is of -^ "^ 



less specific gravity than the 

 dense yolk, it follows that the 

 germinal disc always comes to 

 be uppermost if the egg in its 

 albumen is rolled over, and is 

 hence during incubation always 

 next the warm body of the hen. 



After the egg has traversed Fic - 



the albumen-forming region of 



the Oviduct it receives a thin s.m, shell membrane. In the ct-ntrc at tlu 

 A -fr f pole is seen the germinal <li-< with the " NIK 1- 



layer of a different type of g^..^ n ^^ wM te y clk--Aowtagttaoaflilt 

 secretion which forms a thin 



tough fibrous membrane the shell-membrane (Fig. 191, s.m) and 

 finally on the surface of this is deposited a layer of calcium carbonate 

 which hardens to form the porous egg shell. At the broad end of the 

 " egg " the shell-membrane splits into two layers and as the albumen 

 shrinks in volume during the course of development these two layers 

 separate forming the air space (Fig. 191, a.s) in which air, diffused in 

 through the pores of the shell, accumulates in preparation for the young 

 chick taking its first breath into its lungs. 



In an egg kept under ordinary conditions a similar accumulation of 

 air is brought about by the shrinkage of the white due to evaporation 

 and hence the housewife's dislike to an egg which by an active tilting up 

 of its blunt end when submerged in water shows that its air-space is large. 



As might be expected the segmentation is meroblastic, confined to 

 the protoplasm of the germinal disc. The details of this process, which 

 takes place as the egg passes down the oviduct, need not be gone into 



