But the resemblance goes even further. The ovum is much larger than the sperm-cell, and undergoes a kind of reduction of its nuclear substance; shortly before fertilization the ovum-nucleus ('the germinal vesicle') comes to the surface--just as in the case of animal ova--bursts, and extrudes a part of its substance in the form of a sphere (Fig. 121, 6 and 7). A reduction of the nuclear substance in the male cell has not been demonstrated in all cases, but in one of the _Lithobius_-Coccidia, _Adelea ovata_, the relatively large microgamete (the sperm-cell, Fig. 122, _Mi_) places itself close to one pole of the female macrogamete (the egg-cell) and then divides twice in succession, so that four small cells arise (Fig. 122, _A-C_); of these only one penetrates into the egg-cell (_D_, ♂_K_) and unites with it, the other three come to nought (_D_, _Mi_). What a surprising resemblance this bears to the twofold division of the mother sperm-cell in multicellular animals, through which the number of chromosomes is reduced to half! In the conjugation itself the thread-like chromosomes of the female nucleus are plainly recognizable, while those of the male remain coiled up (Fig. 122, _D_). That the nuclear substance can be separated into chromosomes (ids) even in lowly unicellular organisms was probably first demonstrated by R. Hertwig for _Actinosphærium_, a Heliozoon or freshwater sun-animalcule, then by Lauterborn in regard to Diatoms, by Blochmann for an indigenous Rhizopod, _Euglypha_, and by Ishikawa for the marine _Noctiluca_. Fresh cases have been added in the last decade, so that we can now say that a considerable number of unicellulars, from the ciliated Infusorians and lower Algæ down to the Coccidia and Diatoms, exhibit a germ-plasm composed of ids. These structures behave in the same way as those in higher organisms, and Berger was able to demonstrate, in 1900, in the case of a Radiolarian, their multiplication by spontaneous splitting. [Illustration: FIG. 122. Conjugation of a Coccidium (_Adelea ovata_), after Schaudinn and Siedlecki. _A_, the microgamete (sperm-cell) (_Mi_) has become closely apposed to the macrogamete (_Ma_). _B_, the reduction division of the nucleus of the macrogamete has been effected; _Rk_, directive corpuscles. In the microgamete the first division of the nucleus has begun. _C_, four nuclei in the microgamete, of which three come to nought. _D_, the fourth microgamete-nucleus (♂_K_) has become apposed to the nucleus of the ovum, in which distinct chromosomes are seen.]