SEXUAL REPRODUCTION. 



I 4 I 



VI. Origin of Fertilization. — To understand the origin of the 

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

 That fertilization 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 Maupas and others have so patiently 

 elucidated. 



(a). In the primitive life-cycle exhibited by Protomyxa (see fig. at 

 p. 114), the units which burst forth from the cyst sink down into tiny 

 amoebae, and unite together in numbers to form a composite spreading 

 mass of protoplasm, technically known as a Plasmodium. This is un- 

 doubtedly 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 nucle- 

 ated mass of protoplasm, of composite 

 origin, spreads over the bark in the 

 tanyard. The plasmodial union also 

 occurs as a definite stage in the life- 

 history of the primitive neighbors of 

 Protomyxa, the Monera of Haeckel. 

 Pour the liquid contents or body-cavity 

 fluid of a freshly dredged and still ac- 

 tively living seaurchin into a bowl; the 

 cells which float in it, like blood-cor- 

 puscles in the blood, draw together in 

 clotted masses. Watch the process un- 

 der a microscope, and the formation of 

 a plasmodium is seen. The dying cells 

 fuse into composite masses, just like the 

 units of Protomyxa ; and it is interest- 

 ing 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 fertilize each other articulo mortis. In spite of the objection 

 of Michel and others, that such union, being pathological, is not com- 

 parable 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 pathological processes. Now 

 it is from this primitive union of cells, as illustrated in the lowest organ- 

 isms, that we start in explaining the origin of fertilization. Just as the 

 very beginning of reproduction may be detected in the almost me- 



te&# 



Fig. 47. — Diagrammatic representation of the 

 stages in the origin of fertilization, — I., 

 Plasmodium ; II., multiple conjugation ; 

 III., ordinary conjugation ; IV., conju- 

 gation of dimorphic cells; V., fertiliza- 

 tion of ovum by spermatozoon. 



