MATURATION OF THE EGG. 115 



the surface of the ovary like small shot ; but they have still to 

 pass through the process of maturation, or ripening, before they 

 are ready to be fertilised. This process of maturation concerns 

 the nucleus almost exclusively. 



The nucleus, which at its full size we have seen to be quite 

 half the diameter of the egg itself, begins to shrink ; the 

 nuclear membrane becomes wrinkled, its surface presenting a 

 number of small wart-like projections, so that the whole nucleus 

 has a blackberry-like appearance. Part of the nuclear fluid 

 exudes through the nuclear membrane into the substance of the 

 •egg ; a great part of the nuclear reticulum disappears, or 

 becomes broken up into isolated globules or nucleoli, but a very 

 small part remains in the centre as a slender intricately-coiled 

 thread, the nuclear skein. 



About this time the eggs are discharged from the ovary, the 

 follicles rupturing, and the eggs falling into the body cavity of 

 the frog ; along this they pass forwards, directed partly by con- 

 traction of the muscular body-walls, partly by the action of the 

 ■cilia of the peritoneum, to the mouths of the oviducts, which 

 are situated at the anterior end of the body cavity opposite 

 the roots of the lungs. In the first, or thick-walled, part of the 

 oviduct the eggs acquire gelatinous investments, secreted by 

 glands in its walls. The terminal, or hinder, part of the oviduct 

 forms a thin-walled sac capable of great distension, within 

 which the eggs accumulate in large numbers. Finally the eggs 

 are passed out through the cloaca into water, in which the 

 albuminous investments of the eggs speedily swell up to form 

 the gelatinous mass of the frog's spawn. 



During the discharge of the egg from the ovary, and its 

 passage down the oviduct, further changes occur in its nucleus. 

 The nuclear membrane becomes still further collapsed, and 

 tinally disappears completely; the nuclear fluid and nucleoli 

 ■become distributed through the substance of the egg, and of the 

 original large nucleus the exceedingly minute nuclear skein 

 alone remains. 



This nuclear skein moves from the centre of the egg to its 

 surface, which it reaches opposite the centre of the black hemi- 

 sphere. The skein, previously an irregularly tangled thread, now 

 assumes the definite form and arrangement of a nuclear spindle, 

 :such as may be seen in the nucleus of an epithelial or other cell 

 immediately before division of the cell occurs. 



