216 



THE EVOLUTION THEOIJV 



t'ctnale iiiacrogiuuoti' (the t'j;'i;'-t-oll ) uiid then dixidos twirr in HiU'c-ossiou, 

 SO that four small colls arise (Fij;'- i'-'-, -IC); ol' these only one 

 penetrates into the et^g-ct'll {D, cJ A') and unites with it, the other tlui'e 

 come to nought (7), J//). AVliat a surprising resenihlance this lieai's 

 to the twofold division of the mother sperm-eell in multieellular 

 animals, through which the nundier of chromosomes is reducetl to 

 half! In the conjugation itself the thread-like ehromosomes ol' tlie 

 female nucleus are plainly recognizalile, while those of the male 

 remain coiled up (Fig. 118, /.*). 



That the nuclear substance can be sepaiated into chromosomes 

 (ids) even in lowly unicellular organisms was probably lirst demon- 

 strated by R. Hertwig for ActinottpJuiriurii, a Heliozoon or freshwater 

 sun-animalcule, then by Lauterborn in regard to Diatoms, by Bloch- 

 mann for an indigenous Rhizopod, Eu<jlij/>]n(, and by Ishilvawa for 



Fiii. 122. Conjugal ion of n Cocciilium {Adi-lea unala), iil'toi- Si'liuiuliiiu unci 

 Siedleeki. A, the mici-oganidlo (Hiicrm-oi'll) {Mi) 1ms boc-cnuo closely ai)|)oat'd 

 to the miicrogamete {Ma). B, tlio roduction division of the nuolous of the 

 macrognnioto has been ofl'cctod ; lik, diroclivi' corpuscles. In the niicrogaineto 

 the first division of the nucleus has begun. (', four nuclei in tho mioroganioi.e, 

 of which throe come to nought. P, the lourtli microKaincle-nucleus ^^s K has 

 become apposed to tho nucleus of the ovum, iu \vhi<'li distinct chromosomo.i 

 are seen. 



the marine Noiilhu'd. Fresh cases liavc been added in the last 

 decade, so that we can now say that a considerable number of uni- 

 cellulars, from the ciliated Infusorians and lower Algai down to the 

 Cocciclia and Diatoms, exhibit a germ-plasm eomiio.sed of ids. Tlieso 

 structures behaxe in the same way as those in higher organisms, and 

 Berger was able to demonstrate, in 1900, in tlie case of a Radiolarian, 

 their multiplication by spontaneous splitting. 



From our point of view all this cannot surpi'ise us, since all 

 these organisms, though only single cells, poss(\ss great complexity of 

 structure ; we need only call to mind the extremely line differentiation 

 of structure in numerous ciliated Iid'u.soiiaiis, such as Htcntov, whicli 

 has already becin mentioned, or the bell-animalc\de ( Voiilcd/a) witli 

 its long and peculiarly ciliated gullet, its retractile ciliated disk, its 

 muscular or myophane layer, its spirally retractile stalk with the 



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