148 THE EVOLUTION OF SEX. 



has traced with great care the history of a special kind of protoplasm (what 

 he calls thearchoplasm), which has its centre in either "central corpuscle "(^), 

 and sends out contractile fibrils {/)■, which moor themselves to the nuclear 

 elements. The movements of the latter during the forthcoming first 

 division of the ovum are directly referable to the antagonistic action of 

 these fibrils, and thus we have hints of an intracellular muscularity, the 

 thought of which makes one dumb. 



In the spindle the nuclear elements, still distinguishable in their orderly 

 behaviour as male and female, eventually form what is known as the "equa- 

 torial plate" (VI.), lying across the centre of the spindle. This is a well-marked 

 stage, and one characterised by apparent equilibrium. "It is the resting- 

 stage/ar excellence in the life of the cell. Movement is at an end, a state 

 of stability has set in, and this would continue ad iiifniilii/n, did not a factor, 

 which hitherto has played no part, assert itself and bring about fresh move- 

 ment. This new movement is the longitudinal division of the chromatin 

 elements, an independent expression of life — indeed, a reproductive act — 

 on the part of the nuclear elements." 



»The above short sketch wih show how intricate, and yet at 

 the same time how orderly, are the intimate processes of fer- 

 tihsation. Variations do indeed occur, both in pathological 

 and in apparently normal cases ; but a general constancy is now 

 both clear and certain, not only for many different animals, but 

 also to a certain extent, as Strasburger has shown, for plants. 



One marvellous fact, showing the closeness of union in fer- 

 tilisation, may be briefly re-emphasised. In the double nucleus 

 formed from the union of male and female nuclei, Van Beneden, 

 Carnoy, and others, have shown that both constituents have an 

 equal share. " The one half is purely male, the other purely 

 female, and this is true not only for Ascaris (by Van Beneden) 

 and other thread-worms (by Carnoy), but for representatives of 

 other worm-types, coelenterates, echinoderms, molluscs, and tuni- 

 cates." In division to form daughter-cells (IX., X.), half of each 

 set of constituents goes to either cell, and the dualism is kept up. 

 Furthermore, though hardly yet quite certain, it is most probable, 

 that "of the four chromatin loops observed in the division figure 

 of a daughter-cell, two are derived from the male parent, and two 

 from the female. "" The importance of this fact, in relation to the 

 influence of both parents upon the offspring, is very obvious. 



§ 5. Fertilisation in Protozoa. — In the nascent sexual union observed in 

 many Protozoa, — not, however, as yet in foraminifers or radiolarians, — con- 

 siderable diversity obtains. The individuals which unite may be to all 

 ajipearance similar (to which cases the term conjugation is generally applied), 

 or they may l)e materially dimorphic, as in Vorticella. The union may be 

 permanent, when the two units fuse into one; or it may only be temporary, 

 during which an interchange of elements takes place. In both cases the 

 nuclear elements play an important part, disrupting and reconstructing 



