THE VITAL PROPERTIES OF THE CELL 259 



meet in the centre of the egg, to become surrounded by an 

 aureole of non-granular protoplasm, outside of which there is a 

 radiation sphere, common to them both (sun-like figure and 

 aureole of Fol). 



During the course of the next twenty minutes the egg-nucleus 

 and the sperm-nucleus fuse together to form a single germinal or 

 cleavage nucleus (Fig. 141 I-IV) ; at first they lie close to one 

 another, flattening their contingent surfaces (Fig. 141 IT), until 

 finally the lines of demarcation disappear, so that they unite to 

 form a common nuclear vesicle. In this the substance derived 

 from the spermatozoon may be distinguished for a considerable 

 time as a distinct granular mass of nuclein, which eagerly absorbs 

 staining solutions. 



The fusion of the centrosomes (Fig. 141 I) follows closely on 

 the union of the nuclei. They lie, surrounded by the homo- 

 geneous protoplasmic area, on opposite sides of the cleavage 

 nucleus (Fig. 141 II} ; they then spread themselves out tangen- 

 tial ly upon its surface, assuming the shape of a dumb-bell, and 

 finally divide into halves, which move off in opposite directions 

 (Fig. 141 ///), and travel over one quarter of the circumference 

 of the cleavage nucleus. By means of these circular movements 

 (Fol's quadrille), half of each male centrosome approaches a cor- 

 responding half of a female centrosome ; the plane in which they 

 meet finally intersects at right angles the one in which they were 

 first represented as lying (Fig. 141 IV). Here they fuse together 

 to form the centrosomes of the first division figure. This con- 

 cludes the process of fertilisation, since all further changes are 

 connected with the division of the nucleus. 



b. Eggs of Ascaris megalocephala. Further knowledge of the 

 process of fertilisation may be gained from the study of the eggs 

 of Ascaris megalocephala. Here the spermatozoon penetrates into 

 the egg before the development of the pole-cells (cf. Fig. 127, and 

 the text on p. 237), arriving finally at the centre (Fig. 142 I) ; 

 meanwhile the germinal vesicle, after changing itself, in the 

 manner already described, into the pole spindle, mounts up to the 

 surface of the yolk, and gives rise to several pole cells. Two 

 vesicular nuclei develop, one derived from the nuclear substance 

 of the spermatozoon, which has entered, and the other from one 

 half of the second polar spindle (Fig. 142 I). Egg-nucleus and 

 sperm-nucleus (Fig. 142 17) then approach each other ; in this 

 case, however, the male nucleus is in the centre, whilst the female 



