THE VITAL PHENOMENA OF THE CELL 



193 



as the large number of small granules, which are situated in 

 the yolk, are arranged in rows, passively following the arrange- 

 ment of the protoplasm. After a short time this radiated appear- 

 ance, which is the result of the processes which occur during 

 fertilisation, begins to fade, and to become metamorphosed into 

 two radiated systems, which are found at opposite points of the 

 nucleus. These are small at first, but become momentarily larger 

 and more distinct, until finally they extend all over the whole 

 yolk-sphere, dividing it up into two radiated masses, each arranged 

 around its own attractive centre (Fig. 89). 



A small homogeneous spot can be 

 distinguished in the middle of each 

 radiation from the very beginning ; 

 this spot adheres closely to the nuclear 

 surface, and is free from granules. It 

 contains the centrosome, which, how- 

 ever, cannot be distinguished at all in 

 the living object. 



As the radiations become more dis- 

 tinct and more spread out, the collec- 

 tions of homogeneous non-granular 

 protoplasm in the neighbourhood of 

 the centrosomes become larger, whilst 

 at the same time they gradually re- 

 treat farther and farther apart, carry- 

 ing the poles with them. At this period the nucleus loses its 

 vesicular properties, and assumes the spindle structure which has 

 been described in other objects, but which, on account of its 

 minuteness, cannot be distinguished here during life. In conse- 

 quence, the very characteristic dumb-bell appearance, depicted in 

 Fig. 89, develops in the granular yolk. The two collections of 

 homogeneous protoplasm, enclosing the poles of the division figure, 

 form the heads of the dumb-bell ; the non-granular connecting 

 portion indicates the place where, during the preceding stages, 

 the now invisible nucleus was situated. This has been replaced 

 by the spindle, the ends of which extend right up to the centro- 

 somes. The granular yolk mass is arranged in two radial 

 systems around this homogeneous dumb-bell figure. These sys- 

 tems have been named amphiaster, or double star, by Fol. 



The egg, which at the outset was perfectly round, now com- 

 mences to extend itself longitudinally in the direction of the axis 







Fig. 89. Egg of a Sea-urchin 

 preparing to divide; taken from 

 the living object (from O. Hert- 

 wig, Embryology, Fig. 27). The 

 nucleus is invisible, the dumb- 

 btll figure having taken its place. 



