ORIGIN AXD FORMATION OF THE EGG. 433 



egg, forming a flask-shaped central wliito mass of yolk {IV y. i). 

 It is this part that remains soft in a hard-boiled egg. Between 

 the outer thin layer of white yolk and the central tiask-shaped 

 mass are several other thin concentric layers of a pale hue 

 ( Wy. ii), so that the yolk of an egg really consists of alternate 

 thin white and thick yellow strata. Between the yolk and the 

 shell-membranes at each pole of the egg is a twisted cord — the 

 chalaza (Gh). These two chalazse do not quite touch the 

 .nembranes, but spread out to a slight extent over the vitelline 

 membrane. 'I'hey act as buffers which help to keep the yolk 

 in its place. 



Tlie blastoderm varies according as to whether the egg is 

 unfecundated or fertile. In an unfecundated egg this " spot " 

 is whitish in colour and more or less uniform all over ; there 

 may be very irregular clear spaces in it. In a fertile egg we 

 shall see an opaque rim called the area opac.a ; internal to this 

 a clear space, the area pellucida. In the middle of the latter 

 area is a central white body, formed by the upper neck of the 

 flask-shaped central white showing through. This blastoderm 

 is composed of two layers ; the upper consists of a single row 

 of cells, the lower of a larger number more irregularly disposed. 



The Origin and Formation of the Egg. 



As previously pointed out, the left ovary only is found 

 normally in birds, situated on the left side of the backbone and 

 against the dorsal wall. The appearance of the ovary varies a 

 great deal with age. All we need consider here is the active 

 adult state in which it is found when the hen is laying. At 

 this period the ovary consists of a number of various-sized 

 vascular capsules, in each of which is a primitive ovum (fig. 

 214). The smallest or youngest ova are simple cells. Each 

 ovum contains a nucleus (fig 229, n), otherwise the "germinal 

 vesicle " ; in this latter is the nucleolus {nu) or germinal 

 spot The most recently formed ova are naked cells, and 



2 E 



