192 



THE CELL 



Daring the fourth stage the cell-body divides, and the daughter- 

 nucleus becomes built up again. This, according to van Beneden, 

 takes place in the following manner (Fig. 87) : the four chromatin 



C 



B 



Fig. 87. A A group of four daughter-segments seen from the pole, the swellings at the 

 ends, forming the loops, are especially well marked (after van Beneden and Neyt, PI. VI., 

 Fig. 19). B Reconstruction of the nucleus from the four daughter-segments, diagrammatic 

 (from van Beneden and Neyt, PI. VI., Fig. 20). C Resting condition of the nucleus, seen 

 from the pole (from van Beneden and Neyt, PI. VI., Fig. 21). 



loops (A) absorb fluid, which becomes nuclear sap, out of the 

 protoplasm ; they become saturated with it, as a sponge with 

 water, and thus swell up into thick vesicular bodies (B). The 

 nuclein divides up into granules, which are connected together by 

 delicate threads, which are situated chiefly upon the surfaces of 

 these vesicles. The inner surfaces of these latter come close 

 together and fuse. Thus a vesicular nucleus, irregular in shape, 

 and saturated with nuclear sap, is formed; it is separated from the 

 protoplasm by a membrane, and contains a delicate framework, 

 upon which the chromatin substance is distributed. 



The eggs of Ascaris afford us special advantages for the study 

 of centrosomes and nuclear segments, but the small eggs of 



Echinoderms (Hertwig VI. 30a ; 

 Fol VI. 19a) are also of great use, 

 particularly for observing radia- 

 tion phenomena in the protoplasm 

 of the living cell. More will be 

 said about this later on. 



In the egg-cell of a living 

 Echinodervi, a few minutes after 

 fertilisation (Fig. 88), the small 

 globular cleavage-nucleus is seen 

 to be situated in the centre of the 

 yolk ; it looks like a clear vesicle, 

 and is surrounded by rays of proto- 

 plasm, like a sun with rays of 

 light. This radiation is so dis- 



Fig. 88. Egg of a Sea-urchin just 

 after fertilisation has been completed 

 (from O. Hertwig, Embryology, Fig. 20). 

 Egg nucleus and sperm nucleus are 

 fused to form the cleavage nucleus (fk) 

 which occupies the centre of a proto- 

 plasmic radiation. 



tinct in this object during life, 



