106 DEVELOPMENT OF THE FROG 



vitelline membrane, is formed immediately around it, within 

 the follicle. The mode of origin of the vitelline membrane is 

 not clearly made out, but it seems to be formed from the ovum 

 itself rather than from the follicular epithelium. 



A little later still a layer of black pigment appears on the 

 surface of the ovum ; it is at first irregularly distributed over 

 the whole surface, but as the ovum ripens it becomes restricted 

 to one half or hemisphere. The pigment is contained, and 

 apparently formed, within the ovum itself, but it is not clear 

 how it is formed or what purpose it fulfils. 



B. Maturation of the Egg. 



The eggs have now reached their full size, and project from 

 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, out 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 

 contraction 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 disten- 

 sion, within which the eggs accumulate in large numbers. 

 Finally, the eggs are passed out through the cloaca into 



