8 



EARLY EMBRYOLOGY OF THE CHICK 



the yolk. Except in the neighborhood of the nucleus the active 

 cytoplasm is but a thin film enveloping the yolk. About 

 the nucleus a considerable mass of cytoplasm is aggregated. 

 The region of the ovum containing the nucleus and the bulk of 

 the active cytoplasm is known as the animal pole because this 

 subsequently becomes the site of greatest protoplasmic activity. 

 The region opposite the animal pole is called the vegetative 

 pole because while material for growth is drawn from this 

 region it remains itself relatively inactive. 



young follicle 



germinal epithelium 

 of ovary 



white yolk 



yellow yolk 



cellular (granular) 

 zone of follicle 



FIG. i. Diagram showing the structure of a bird ovum still in the ovary. 

 (Modified from Lillie, after Patterson.) The section shows a follicle containing 

 a nearly mature ovum, together with a small area of the adjacent overian tissue. 



Enclosing the ovum is a thin non-cellular membrane, the 

 vitelline membrane, which is a secretory product of the cyto- 

 plasm of the ovum. Outside the vitelline membrane and very 

 difficult to differentiate from it, is another secreted membrane 

 the zona radiata, so called because of its delicate radial stria- 

 tions. Immediately peripheral to the zona radiata is an invest- 

 ment of small polygonal cells, the cellular or " granular" zone 

 of the follicle, which is in turn enclosed in a highly vascular 

 coat of connective tissue, the theca folliculi. The nutriment 

 for the growing ovum is supplied by the mother from the prod- 



