PART I 



THE EARLY DEVELOPMENT TO THE END OF 



THE THIRD DAY 



CHAPTER I 



THE EGG 



The parts of a newly laid hen's egg are the shell, shell-mem- 

 brane, albumen, and yolk. In an egg that has been undisturbed 

 for a short time the yolk floats in the albumen with a whitish 

 disc, the blastoderm about 4 mm. in diameter, on its upper sur- 

 face. If the yolk be rotated, it will return to its former position 

 in a few minutes, owing to the slightly lower specific gravity of 

 the hemisphere containing the blastoderm. The blastoderm is 

 the living part of the egg, from which the embryo and all its 

 membranes are derived. It is already in a fairly advanced stage 

 of development when the egg is laid. The yolk and blastoderm 

 are enclosed within a delicate transparent membrane (vitelline 

 membrane) which holds the fluid yolk-mass together. We may 

 now consider some details of the structure and composition of 

 the parts of the egg. 



The shell is composed of three layers: (1) the inner or mam- 

 millary layer, (2) the intermediate spongy layer, and (3) the 

 surface cuticle. The mammillary layer consists of minute cal- 

 careous particles about 0.01-0.015 mm. in diameter welded to- 

 gether, with conical faces impinging on the shell-membrane; the 

 minute air-spaces between the conical inner ends of the mammillae 

 communicate with the meshes of the spongy layer, which is sev- 

 eral times as thick, and which is bounded externally by the ex- 

 tremely delicate shell cuticle. The spongy layer consists of 

 matted calcareous strands. The shell cuticle is porous, but 

 apparently quite structureless otherwise. The cuticular pores 

 communicate with the mesh-work of the spongy layer; thus the 

 entire shell is permeable to gases, and permits of embryonic 

 respiration, and evaporation of water. 



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