I.] THE FROG. 155 



rarely seen, as it only occasionally lies in the 

 plane of section. 



ii. Examine under a high power. 



a. The smaller ovisacs. 



a. The ovum ; the central large cell ; its proto- 

 plasm, uniformly finely granular and deep- 

 staining. 



/?. Germinal vesicle; often irregular in outline; 

 germinal spots diffused or arranged peri- 

 pherally in a single row. 



y. Ovarian follicle- a single-layered epithelial 

 investment for the ovum. 



b. Compare the larger ovisacs 



a. Protoplasm of ovum. Peripherally, black and 

 pigmented, otherwise converted, for the most 

 part, into an immense aggregate of small ellip- 

 soidal or irregular refractive yolk granules. 



fi. Germinal vesicle. Look for specimens in 

 which this is visible ; the germinal spots are 

 mostly, if not wholly, aggregated in the middle 

 of the same. 



y. Ovarian follicle ; still recognizable, but gene- 

 rally flattened. 



8. Vitelline membrane; obvious as a thin faint 

 line, interposed between the ovum and the 

 follicle. Look for it in ova whose investing 

 structures may have been ruptured. 



c. The germinal epithelium ; a cellular mass imme- 

 diately beneath the investing membrane of the 

 ovary. Look for young ovisacs still in connection 

 with it. 



