XIV 



ELEMENTS OF VERTEBRATE EMBRYOLOGY 



457 



towards the external opening in its passage down the oviduct. Along 

 the long axis of the mass of albumen — from the vitelline membrane to 

 each pole — there passes a denser less transparent strand of albumen 

 known as the ehalaza (Fig. 191, ch). The chalazae represent early formed 

 portions of the albumen which preceded and followed after the egg in 

 its passage along the oviduct. During this passage the egg rotates 

 slowly on itself and the inner ends of the chalazae being attached to the 

 vitelline membrane these structures are given a characteristic twist in 

 opposite directions. The chalazae serve to keep the egg in the midst of 

 the albumen while allowing it to rotate freely about the long axis of the 

 mass of albumen. As the germinal disc is in a position which is practically 

 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 

 the albumen-forming region of 

 the oviduct it receives a thin 

 layer of a different type of 

 secretion which forms a thin 

 tough fibrous membrane — the shell-membrane (Fig. 191, s.ni) — 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 



Fig. igi. 



Egg of the Fowl, which has been laid without being 

 fertilized, a.5, air-space ; alb, albumen ; cA, ehalaza ; 

 s.m, shell membrane. In the centre — at the apical 

 pole — is seen the germinal disc with the " Nucleus of 

 Pander" — a mass of white yelk — showing through it. 



