MATURATION OF THE OVUM. 



965 



Between the ectoblast and the hypoblast there now develops, from 

 the primitive streak, as a product of the cellular hyperplasia of the 

 ectoblast, the mesoblast, which, growing peripherally, insinuates itself 

 between the two former. The three germinal layers (in birds) in their 

 growth arrange themselves according to size in such a manner that the 

 uppermost is the largest, the middle the next in size, and the under- 

 most the smallest. All three grow at the periphery. As the middle 



Transition from close 

 to loose skein. 



Polar radiation (achromatic substance) 



Convexity of 

 loops. 



Polar field. 



Longitudinal di 

 vision of chro- 

 matic substance 



Loose skein. 



Segmented skein with 

 24 loops. 



Polar radiation. 



Polar radiation. 



Longitudinal splitting 

 of loops. 



Polar radiation. 

 Terminal stage of mother-star. 



Loops arranged in 

 equatorial plates. 



Polar radiation. 



Aster. 



Polar body. 



Connecting threads. 



Loops. 



New nucleus. 



Loops 



MeUikinrsis. 



Polar body. 



Loops. 



New nucleus. 



Completed division. 

 FIG. 375. Stages of Nuclear Division (after Rabl). 



layer develops into vessels, its border is always easily recognizable 

 from the sinus the future terminal vein. The border of the upper 

 layer encloses the yellowish-white, wavy vitelline area; the border of the 

 middle layer, the vascular area ; the embryo lies in a portion of the 

 pellucid area that is dumbbell-shaped and clear as glass. As all three 

 plates, finally, surround the entire yolk, their borders come in contact 

 with the pole of the yolk lying opposite to the embryo. 



