216 



THE EA'OLUTIOX THEORY 



female macroo-ainete (the egg-cell) and then divides twice in succession, 

 so that four small cells arise (Fig. i23, A-C): of these only one 

 penetrates into the egg-cell {D, (^ K) and unites with it, the other three 

 come to nought (7), 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. ii8, D). 



That the nuclear substance can be separated into chromosomes 

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

 strated by R. Hertwigfor Actinosj^hcerium, a Heliozoon or freshwater 

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

 mann for an indigenous Rhizopod, Euglyplia, and by Ishikawa for 



Fig. 122. Conjugation of a Coccidium {AcMea ovaia , after Schaudinn and 

 Siedlecki. A, the niicrogamete ;sperm-ctll i {Mi) has become closely apposed 

 to the macrogamete {Ma] . B, the reduction division of the nucleus of the 

 niacrogamete has been effected ; Bk, 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 (sK has 

 become aj^posed to the nucleus of the ovum, in which distinct chromosomes 

 are seen. 



the marine Xoct'duvu. Fresh cases have been added in the last 

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

 cellulars, from the ciliated Infusorians and lower Alga3 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. 



From our point of view all this cannot surprise us, since all 

 these organisms, though only single cells, possess great complexity of 

 structure ; we need only call to mind the extremely fine diff*erentiation 

 of structure in numerous ciliated Infusorians, such as Sfentor, which 

 has already been mentioned, or the bell-animalcule (Vortlcella) with 

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

 muscular or myophane layer, its spirally retractile stalk with the 



