PA a on 
LHE 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 (8) the 
surface cuticle. The mammillary layer consists of minute cal- 
‘rareous 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 mammillz 
communicate with the meshes of the spongy layer, which is sey- 
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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