230 THE CHICK. 



structure, with one or more nucleoli : it is now spoken of as 

 the germinal vesicle ; and the establishment of the germinal 

 vesicle, together with the marked increase in size of the ovum, 

 owing to the accumulation of yolk-granules within it, mark the 

 conversion of the primitive ovum, which occurs in both sexes 

 alike, into the permanent ovum characteristic of the ovary of 

 the hen bird. 



As the egg increases in size it forms a swelling on the sur- 

 face of the ovary, which rapidly becomes more prominent. A 

 vitelline membrane is formed round the egg, between it and 

 the follicular epithelium, and apparently derived from the egg 

 itself. The follicular epithelium, with the outer wall of the 

 ovary, form a vascular capsule, investing the egg. 



The accumulation of yolk-granules within the egg continues 

 until this has reached its full size. From an early stage, a 

 difference may be noticed between white yolk-spheres, and yellow 

 yolk-spheres ; the former consisting of minute vesicles, each 

 containing a highly refracting body; while the latter, which 

 are apparently derived from the white yolk-spheres, are much 

 larger bodies, filled with numerous bright, highly refracting 

 granules. 



In the fully-formed egg the white and yellow yolk-spheres 

 are arranged in a very definite manner. The yellow yolk makes 

 up the greater part of the bulk of what we call the yolk of the 

 egg ; the white yolk-spheres forming, (i) a somewhat flask-shaped 

 plug in the centre of the yolk, with a neck reaching to the 

 surface at the germinal disc ; (ii) a thin superficial layer invest- 

 ing the whole exterior of the yolk, immediately below the vitelline 

 membrane ; (iii) a series of thin concentric shells between the 

 surface and the central plug, the spaces between the successive 

 shells being occupied by the yellow yolk. 



2. Maturation of the Egg. 



The ripening of the egg is accompanied by changes in the 

 nucleus, which are as yet only imperfectly known. 



The nucleus, or germinal vesicle, during the growth of the 

 egg, is large and vesicular, and occupies a position at or close 

 to the centre of the egg. As the egg ripens, the nucleus moves 

 towards the surface, where it lies just beneath the vitelline 

 membrane, in]^a small lenticular patch, the germinal disc, which 



