FOKMATION AND MATUKATION OF THE EGG. 97 



while it is still in the ovary, shortly before it reaches its full 

 size, and the successive stages are shown in Fig. 45. 



The whole nucleus shrinks considerably, becoming reduced 

 to less than half its former diameter. This shrinking is accom- 

 panied by exudation of part of the nuclear fluid, through the 

 nuclear membrane, into the protoplasm of the cell-body (Fig. 

 45, A, UH), where it forms a fluid layer surrounding the nucleus: 

 at the same time the nuclear membrane becomes wrinkled, 

 its surface, which was previously smooth, becoming raised into 

 little wart-like projections, so as to present an appearance some- 

 thing like a blackberry (Fig. 45, A)- 



Within the nucleus a number of the larger chromatiii 

 granules, or nucleoli, remain close to the nuclear membrane, 

 often lying within the wart-like protuberances ; a number of 

 others, chiefly smaller ones, collect towards the centre, where 

 they surround a clear region in which lie a number of exceedingly 

 minute chromatin granules. These latter are at first scattered 

 irregularly, but soon run together to form moniliform threads of 

 extreme slenderness, which interlace and unite to form a minute 

 nuclear skein (Fig. 45, A). 



About the time of discharge of the egg from the ovary further 

 changes occur, which are apparently stimulated by the act of 

 copulation. The nuclear membrane disappears completely ; and 

 its contents, the nuclear fluid and nucleoli, become distributed 

 through the yolk. The only part of the egg nucleus which 

 persists is the minute nuclear skein : this moves towards the 

 surface of the egg, and takes up a position at the upper or black 

 pole of the egg, immediately below its surface (Fig. 45, B) ; 

 here it lies in a lenticular patch, which is rather more fluid and 

 more transparent than the rest of the yolk, and is separated 

 from this by an ill-defined capsule of pigment, prolonged to- 

 wards the centre of the egg in the manner shown in the figure. 



The nuclear skein (Fig. 45, B, UG), now assumes a spindle 

 form, and lies at first with its long axis tangential to the surface 

 of the egg. Shortly afterwards the spindle turns so that its axis 

 becomes radially situated, one of its poles being at the surface 

 of the egg, and the other directed towards the interior ; it then 

 divides transversely into two parts, of which one (Fig. 45, C, UG), 

 remains within the egg, while the other (Fig. 45, C, PB), is 

 extruded as the first polar body. Shortly before the formation 



H 



