"^^[Ca} 



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, uDd 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 embrj^o 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. 



17 



