150 THE EVOLUTION OF SEX. 



nuclear bodies divide as usual ; the large individual ceases to feed, and 

 hermetically closes its mouth, like an ovum when fertilised. The small 

 individual is gradually absorbed by the larger, as sperm by ovum ; and in 

 an intricate but orderly fashion a mixed nucleus results from the fusion of 

 the para-nuclear elements of the two. The adult then begins to feed, to 

 divide, and so on, as usual. Here then there is (a) incipient dimorphism, (/^) 

 absorption of smaller by larger, and [c) intimate nuclear union, — facts which 

 we have already emphasised in the fertilisation of multicellular animals. 



§ 6. Origin of Feriilisatioii. — To understand the origin of 

 the union of sex-cells, attention must still be concentrated on 

 the Protozoa. That fertilisation really occurs at that low level 

 in a highly complex fashion, we have just seen. It is necessary, 

 however, to note the steps which lead up to what ?^Iaupas and 

 others have so patiently elucidated. 



{a) In the primitive life-cycle exhibited by Protoinyxa (see 

 fig. at p. 120), the units which burst forth from the cyst sink 

 down into tiny amcebae, and unite together in numbers to form 

 a composite spreading mass of protoplasm, technically known 

 as 2i Plasmodium. This is undoubtedly a very primitive union 

 of cells, yet it occurs at very diverse levels in the organic series. 

 It is more or less familiar in the " flowers of tan," one of the 

 lowly Myxomycetes, where a nucleated mass of protoplasm, 

 of composite origin, spreads over the bark in the tan-yard. 

 The plasmodial union also occurs as a definite stage in the hfe- 

 history of the primitive neighbours of Frofomyxa, the Monera 

 of H^ckel. Pour the liquid contents or body-cavity fluid of a 

 freshly-dredged and still actively living sea-urchin into a bowl; the 

 cells which float in it, like blood-corpuscles in the blood, draw 

 together in clotted masses. Watch the process under a micro- 

 scope, and the formation of a plasmodium is seen. The dying 

 cells fuse into composite masses, just like the units oi Protomyxa ; 

 and it is interesting to observe that, though they are dying, the 

 union provokes a brief but intense renewal of amoeboid activity. 

 To forestall our point, they as it were fertilise each other in 

 articulo mortis. In spite of the objection of IMichel and others, 

 that such union, being pathological, is not comparable to the 

 multiple conjugation normal to the myxomycete, we maintain 

 the distinct analogy between the plasmodium formation in 

 myxomycetes and that exhibited by the cells in the body-cavity 

 fluid of many animals, and regard this as so much additional 

 evidence of the profound unity of the normal and the patho- 

 logical processes. Now it is from this primitive union of cells, 

 as illustrated in the lowest organisms, that we start in explain- 



