ANNELIDA. 



73 



old constituents, (6) addition of new constituents from another individual. 

 In Vorticella there is in addition dimorphism, the conjugating elements 

 differing in size and activity. This is a form of physiological division of 

 labour, for a small active cell is best fitted for the mere transfer of nuclear 

 material, and a large cell is a convenient storehouse of nutriment for 

 developmental needs. This is the meaning of the striking differences 

 between sperm and ovum in the sexual reproduction of the Metazoa. 

 Again, in the maturation of both ova and sperms the elimination of nuclear 

 material just described for ciliate Protozoa is paralleled. This is the 

 import of the polar cells or bodies which are separated off from ripening 

 ova. An ovum is commonly invested by a cell- wall, the vitelline membrane, 

 within which are the protoplasm and nucleus (germinal vesicle). The pro- 

 toplasm (vitellus) consists of a delicate living network, the meshes of which 

 enclose different substances, especially nutritive granules or food-yolk, the 

 amount of which varies with the size of the ovum. The nucleus is covered 

 by a delicate nuclear membrane, and it consists of a modification of pro- 

 toplasm roughly divisible into (a) achromatin, staining with difficulty, 

 and traversed by (&) an easily stained convoluted thread or network of 

 chromatin, part or all of which may be condensed into a rounded nucleolus 

 (germinal spot). Fig. 21 represents the stages in the formation of polar 



Fig/21. FORMATION or POLAR CELLS IN A STARFISH (Asterias glacialis). 

 A.K, after Fol ; L, after O. Hertivig. A, Ripe ovum, with excentric 

 germinal vesicle and spot. B-D, Gradual metamorphosis of germinal 

 vesicle and spot, as seen in the living egg, into two asters. F, Forma- 

 tion of first polar cell and withdrawal of remaining part of nuclear 

 spindle within the ovum. G, Surface view of living ovum, with first 

 polar cell. H, Completion of second polar cell. I, A later stage, showing 

 the remaining internal half of the spindle in the form of two clear 

 vesicles. K, Ovum, with two polar cells and radial striae round female 

 pronucleus, as seen in the living egg. [E, F, H, I], From picric acid 

 preparations. L, Expulsion of first polar cell. 



cells in the case of a starfish (Asterias glacialis). In A are seen the 

 excentric germinal vesicle and spot, which become less distinct in B, C, 

 and D. A "nuclear spindle" is then formed, consisting of threads along 

 which half the chromatin passes to one end and half to the other. At each 

 end of the spindle a small "central corpuscle" or centrosoma appears, 

 which is the centre of a radiating sun-like figure, and appears to be the seat 

 of a special sort of protoplasm (archoplasm) that determines the division 

 and movement of the chromatin. The spindle now places itself vertically 



