Archive image from page 34 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 w 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 low
Archive image from page 34 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 w 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 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
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