86o LIFE : OUTLINES OF GENERAL BIOLOGY 



nuclear reduction, by a minute motile microgamete, the occurrence 

 is evidently not very different from the fertilisation of a Metazoan 

 egg-cell by a sperm-cell. And the anticipation becomes even more 

 marked when a Protozoon colony, as in some Radiolarians, produces 

 special dimorphic gametes which unite to form a zygote — the 

 starting-point of another colony. In short, there remain many 

 indications of the primitive steps towards multiplication on the 

 one hand and sex on the other. 



THE MULTICELLULAR BODY.— It was a step of far-reaching 

 importance when many-celled organisms began, when Metazoa and 

 Metaphytes appeared on the stage previously confined to the Protozoa 

 and Protophytes. What light do existing forms throw on the bridging 

 of the gap between unicellulars and multicellulars, which Agassiz 

 called "the greatest guLf in organic nature"? The clue is probably 

 to be found in those Protozoa and Protophytes which form loose 

 colonies of cells. Thus the spherical green colony called Vol vox, of 

 1,000 or even 10,000 cells according to the species, is a pinhead-like 

 "body", often formed by the division of a fertilised macrogamete, 

 practically an egg-cell. It inclines in some ways to the plants, and 

 is often claimed by the botanists ; but there are also many emphatic 

 Protozoon colonies, e.g. in several orders of Radiolarians, and some 

 of them are the size of split peas. What has happened in such cases 

 is that the daughter-units, formed by division of the original cell, 

 have remained associated, instead of drifting apart in individual 

 completeness. Out of this weakness — ^if it was a weakness — strength 

 arose, the strength of animals with a body. 



Another possible mode of origin is by repeated division of the 

 nucleus, without corresponding division of the cytoplasm, and there 

 are many multinucleate Protozoa, such as the giant Amoeba 

 (Pelomyxa), large enough to be cut into two with a scalpel, and the 

 beautiful ciliate Opalina that lives in the frog's intestine. If some 

 cytoplasm should gather round each of these nuclei, as happens in 

 the division of many ova which pass through a "syncytial" stage, 

 a multicellular body might result. 



It is probable that various experiments were made in body- 

 making. Thus in some simple colonial Algae, like Gonium, a platelet 

 of cells is formed — the so-called placula; in other cases, e.g. the 

 rather dubious Proterospongia, the incipient body is an irregular 

 blob or parenchymula, with amoeboid cells in an internal gelatinous 

 matrix and collared flagellate cells round the margin. Yet in all 

 probability the successful tentatives in body-making took the form 

 of hollow balls of cells, one layer thick (monoblastic), whence, at a 

 subsequent stage, two-layered (diploblastic) gastrula-like types 

 arose. 



Here some reference should be made to the small and somewhat 



