FIG. 56.—Functionating female generative organs of a hen. 1, Ova in process of formation of yolk. 2, Stigmal line at which point the capsule ruptures when ovum is mature. 3, The funnel end of the oviduct. 4, The oviduct torn loose and laid to one side, the albumin-secreting portion. 5, The shell membrane secreting portion. 6, The albumin. 7, The yolk. 8, The shell-secreting portion. 9, The cloaca. 10, The rectum. ] _Structure._—The ovary contains very vascular cellulofibrous tissue. The ovum as it develops is attached to the ovarian body by means of a delicate white fibrous pedicle. When the yolk is mature it escapes from the enveloping fibrous capsule by a cleavage of the capsule. The cleavage line is called the _stigmen_ (Fig. 56, No. 2). The yolk is surrounded by a very delicate membrane called the _vitelline membrane_. The empty sac now shrinks and finally disappears. =The Egg.=—The principal divisions of the egg are the yolk, the albumin outside of the yolk content, the shell membranes and the shell (Fig. 58). As stated, the _yolk_ is formed in the ovary, leaving the other three portions to be formed in the oviduct. [Illustration: FIG. 57.—Section through the ovary of the hen. _a_, The ova. _d_, An ovum beginning to receive deposits of yolk material. _c_, Ova farther advanced in yolk formation. ] The _albumin_ may be subdivided into, first, a thin layer of albumin lying close around the yolk; second, a thick layer of albumin lying at the outer periphery; and third, a modification of the albumin called the chalazæ. The chalazæ are twisted, dense cord-like structures at either pole of the yolk, one end of which is adherent to the vitelline membrane and the other to the inner membrane surrounding the albumin. The chalazæ thus act as stays to this structure, which is carrying a delicate burden, the blastoderm. The _shell membrane_ consists of two layers, an inner delicate, and an outer thicker layer. When the egg is just laid these two membranes are in all parts closely adherent to each other, and the egg content completely fills the shell cavity. As soon as the content cools there is a slight contraction; the two shell membranes separate at the large end of the egg, forming an air cell which gradually enlarges as the evaporation of liquid through the pores of the shell takes place. [Illustration: FIG. 58.—A diagram of the parts of the egg. _a_, The blastoderm. _b_, The shell. _c_, The outer shell membrane. _d_, The inner shell membrane. _e_, The air-sac at the large end of the egg. _f_, The albumin. _g_, The chalaza. _h_, The yellow layers of yolk. _i_, The white layers of yolk. _k_, The flask-shaped portion of white yolk. _l_, The vitelline membrane. ]