DEVELOPMENT OF THE EMBRYO 



443 



nucleus as well as throughout the cytoplasm, except in the peripheral 

 region. This- region remains comparatively free from yolk. At the point 

 where the ovum or follicle is attached (Fig. 256), there is a thicker por- 

 tion in the periphery known as the germinal disc or spot. As soon as 

 the ovum reaches a diameter of about five-tenths of a millimeter, the 

 nucleus migrates into the germinal disc, where it remains as long as the 

 egg continues in the ovary. An important point to remember is that 

 the animal pole of the ovum is toward the attached surface, that is, at 

 the point where the nucleus is located. 



From this time onward, the yolk accumulates very rapidly. The 

 surface of the ovum is in the form of zona radiata (Fig. 256, B), in w r hich 

 there are many pores through which nutritive substances may easily 

 diffuse from the follicle cells. These follicle cells may therefore be called 

 nurse cells. 



The manner in which we estimate the rate of growth in all final 

 stages of the hen's egg is by the arrangement of the concentric layers 

 of white and yellow yolk. This is possible, because layers mark daily 

 additions in the formation of yolk or food-matter. 



When the follicle has completed its growth, it becomes somewhat 

 membranous. Directly opposite its point of attachment there are very 



Fig. 255. 



Young Oocyte surrounded by a single 

 layer of Follicular Cells. (Van der 

 Stricht.) 



Showing attraction-sphere, centro- 

 some, and mitochondria. 



Corona radiata. 

 Zona pellucida. 

 Germinal spot. 



Germinal vesicle or 

 nucleus. 



Fig. 256. 

 A, ripe Graafian follicle. B, ovum. 



few blood vessels, and it is at this point that a modification takes place 

 in the form of a band appearing, known as the cicatrix. It is at the cicatrix 

 that the follicle ruptures to permit the escape of the egg into the oviduct. 

 The nucleus lies flat against the vitelline membrane, and becomes 

 very large just before the egg leaves the ovary. It is then called a 

 germinal vesicle, because the chromatin condenses, which leaves the 

 nucleus appearing as a large clear spot. The nuclear wall now breaks 



