468 DEVELOPMENT OF THE CHICK. 



II. GENERAL DESCRIPTION. 



The hen's egg is formed by the union of the ' mature ' 

 egg (that is, the egg-cell after attaining its full size and form- 

 ing its polar cells by unequal division), and the ' mature ' 

 spermatozoon that is, one of the sperm-cells formed by the 

 final equal division and differentiation of a sperm mother- 

 cell. These two cells when ready to unite are termed 



1 gametes,' and when united they form a * zygcte.' Hence, 

 strictly speaking, the hen's egg or zygote is a double structure 

 composed of both male and female nucleus and protoplasm. 



All the ova of a hen are produced from her left ovary. 

 The right ovary and oviduct disappear or shrivel early in life. 

 Good ' layers ' produce 100 eggs or more in a season, together 

 with many more immature ova which are absorbed towards 

 the end of the egg-laying period. The ovary then diminishes 

 greatly in extent, and the oviduct shrinks from being a tube 



2 feet long and \ inch wide, to one 6 inches long and y 1 ^ inch 

 thick. In order to see the mature ovary, a hen or pigeon 

 should be examined as on p. 457. The ripe ova are then 

 seen to be enclosed in vascular follicles, and the whole organ is 

 in a state of intense anabolic activity. 



1. Segmentation (cleavage) of the Egg. 



The egg of a bird consists of cytoplasm distended very 

 greatly by concentric layers of white and yellow yolk, except 

 for a lens-shaped area in which the nucleus lies. This region 

 appears as a white disc when viewed from above, and is 

 known as the ' germinal disc,' as it gives rise to the greater 

 part of the bird. The early segmentation is confined to this 

 area. It begins during the passage of the egg down the 

 oviduct, and is completed by the time the egg is laid. The 

 first step in the process is the appearance of two vertical, 

 shallow furrows, at right angles to one another, which penetrate 

 the surface of the disc, but do not extend to the bottom of the 

 cytoplasmic disc. Four radial fissures then appear about 

 midway between the former furrows. These are again 



