8 INTRODUCTION. 



question have been studied most completely in the eggs of 

 Ascaris megalocephala, a thread-worm found living parasitically 

 in the horse ; and in the eggs of certain Echinoderms. In 

 Vertebrates they have not as yet been followed in such detail, 

 but all the more important phases have been seen to occur in 

 the eggs of frogs and other Amphibians, and several of them in 

 the eggs of rabbits and other Vertebrates. The changes appear 

 to be essentially the same in all animals in which they have been 

 observed. 



The process of maturation concerns the egg nucleus, or 

 germinal vesicle, almost exclusively; and the principal stages 

 are as follows. The nucleus, which prior to maturation is of 

 large size, with a well-developed nuclear membrane and reticu- 

 lum, begins to shrink ; the nuclear membrane becomes wrinkled, 

 so that the surface of the nucleus presents an irregular warty 

 appearance (Fig. 1, A). 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, a very small part alone remain- 

 ing as a slender intricately-coiled thread, the nuclear skein. 



The nuclear membrane now shrinks still further, and finally 

 disappears completely; the nuclear fluid and nucleoli become 

 distributed through the substance of the egg, and of the original 

 egg-nucleus all that now remains is the minute nuclear skein 

 (Fig. 1, B). 



The nuclear skein, which was at first placed centrally, or 

 more or less excentrically, now moves to the surface of the egg. 

 The skein, previously an irregularly tangled thread, assumes the 

 definite form and arrangement of a nuclear spindle, such as is 

 seen in the nucleus of an epithelial or other cell immediately 

 before division of the cell occurs ; it then divides into two equal 

 parts, one of which remains within the egg, while the other is 

 extruded from it as the first polar body (Fig. 1, C). After a 

 brief pause the half of this nuclear spindle that has remained 

 within the egg again divides into two equal parts, one of which 

 is extruded as the second polar body, while the other remains 

 within the egg, and is known as the female pronucleus (Fig. 

 1, D). The formation of the female pronucleus, by the separation 

 and extrusion of the two polar bodies, completes the process of 

 maturation. 



