STRUCTURE OF THE OVUM. 7 



contents are extruded (fig. 5, I). The striae in tbe membrane are believed to 

 be minute pores, and are supposed, while the ovum is yet within the Graafian 

 follicle, to permit the passage of granules of nutrient material into the interior of 

 the ovum. After the ovum is discharged from the follicle, the spermatozoa may 

 perhaps find their way into the ovum through these pores. According to Retzius 

 the protoplasm of the ovum is united with the follicle-cells by fibres which pass 

 through the pores of the zona. 



Immediately surrounding the zona radiata, as the ovum lies within the mature Graafian 

 follicle, is a thin stratum of granular substance, probably deposited upon the exterior of the 

 ovum by the innermost cells of the discus proligerus, which immediately encircle the ovum 

 within the follicle. When the Graafian follicle bursts and the ovum is set free, this granular 

 material appears to imbibe water, and, as is specially noticeable in the ovum of the rabbit, 

 swells up into a clear gelatinous envelope, which has been termed, from a possible homology 

 with the white of the bird's egg, the albumen. But in the mammal this structure has not 

 the nutritive importance to the embryo which is possessed by the corresponding formation 

 in the bird, and it disappears during the passage of the ovum down the Fallopian tube. 



The substance of the ovum within the tunica radiata is known as the vitellus 

 or yolk (fig. G, vi). It is a soft semi-fluid substance, composed mainly of proto- 

 plasm, which is filled with globules and granules (yolk-granules) of different 



Fig. 5. OVARIAN OVUM OP A MAMMIFER. (Allen Thomson. ) 



a, the entire ovum, viewed under pressure ; the granular cells have been removed from the outer 

 surface, the germinal vesicle is seen in the yolk substance within ; b, the external coat or zona burst by 

 increased pressure, the yolk protoplasm and the germinal vesicle having escaped from within ; c, germi- 

 nal vesicle more freed from the yolk substance. In all of them the macula is seen. 



Fig. 6. OVUM OF THE CAT ; HIGHLY MAGNIFIED. SEMI-DIAGRAMMATIC. (E. A. S.) 



zp, zona pellucida, showing radiated structure ; vi, vitellus, round which a delicate membrane is 

 seen ; yr, germinal vesicle ; gs, germinal spot. 



sizes, but all small, and possessing a high index of refraction. Examined in the 

 fresh condition, the protoplasm between the granules looks perfectly clear and 

 structureless, but after treatment with suitable reagents, ifc may be seen to consist of 

 a fine reticulum, which is especially fine and close near the periphery of the ovum, 

 and also around the germinal vesicle, at which places the yolk granules are in less 

 amount than elsewhere. The substances which occur within an ovum other than 

 the nucleus and protoplasm, may, as in cells generally, be collectively designated 

 " deutoplasm " ; they are regarded as furnishing a supply of nutrient matter to the 

 protoplasm during the earlier stages of development. 



Embedded in the protoplasmic vitellus, usually eccentrically, is a large spherical 



