16 



THE CELL AND PROTOPLASM 



chromosomes differ in different species of 

 animals and plants but are constant for 

 each species. 



The mitotic figure (amphiaster, Fol 

 1887) is a marvellous apparatus for the 

 exact division of each individual chromo- 

 some. The vesicular nucleus is plainly a 

 compound structure, in some cases com- 

 posed of chromosomal vesicles each con- 

 taining one chromosome and it could not be 

 divided with exact equality in any other 

 way. The chromosomes are thus persistent 

 cell organs, and all parts of chromosomes, 

 such as chromomeres, chromioles and genes, 

 are self-perpetuating by individual growth 

 and division. 



At the poles of the amphiaster. Van 

 Beneden described polar corpuscles, in 

 1874, and attraction spheres, in 1883. Both 

 Van Beneden and Boveri found, in 1887, 

 that these central corpuscles or centrosomes 

 are persistent, self-perpetuating cell or- 

 gans. In 1895 Boveri and others found 

 that the centriole within the centrosome is 

 a persistent organ. In 1894, Heidenhain, 

 and, in 1896, Kostanecki regarded polar 

 rays and spindle fibers as persistent organs, 

 but sufficient evidence of this is lacking. 

 Altmann maintained, in 1890, that certain 

 granules in the cytoplasm (his hioMasts), 

 which are now called mitochondria, are 

 "elementary organisms" comparable to 

 bacteria and capable of growth and divi- 

 sion, but this view is not generally held at 

 present. 



Every persistent part of the cell is self- 

 perpetuating by the process of growth and 

 division. This is known as the principle of 

 "Panmerism" (0. Hertwig). The long 

 disputes regarding the origin of cells was 

 finally concluded with the establishment of 

 the fact that all cells come by division from 

 preceding ones, as was neatly expressed, in 

 1859, in the dictum of Virchow, "Omnis 

 cellula e cellula." In 1884, Strasburger 's 

 demonstration of this principle of panmer- 

 ism in the case of the nucleus led to the dic- 

 tum, ' ' Omnis nucleus e nucleo. ' ' The work 

 of Rabl, in 1885, of Boveri, in 1887, and of 

 Van Beneden, in 1887, proved its truth in 

 the case of chromosomes, "Omnis chromo- 

 soma e chromosoma," and the work of the 



last two named authors extended it to 

 centrosomes, "Omnis centrosoma e centro- 

 soma," and to central bodies, later named 

 by Boveri, in 1895, centrioles, "Omnis 

 centriola e centriola. " In 1890, Altmann 

 claimed that his granules or bioblasts were 

 panmeristic, and, in 1894, Heidenhain held 

 a similar view regarding spindle fibers, but 

 probably these are not self-perpetuating. 

 There are undoubtedly still others, not in- 

 cluded in this list, which are self-perpetuat- 

 ing units of protoplasm. This applies par- 

 ticularly to the ultramicroscopic units of 

 life and heredity, which have been postu- 

 lated by many students. 



Long ago, in 1861, Briicke maintained 

 that because of its growth by intussuscep- 

 tion protoplasm must be composed of ultra- 

 microscopic units capable of assimilation, 

 growth and division, and he called these 

 units "the smallest living parts." Sup- 

 posed logical necessity led Herbert Spen- 

 cer, in 1866, to postulate his "physiological 

 units," each endowed with all the essential 

 properties of life. As is well known, in 1868 

 Darwin assumed the existence of different 

 kinds of "gemmules" in his "provisional 

 hypothesis of pangenesis" to account for 

 phenomena of heredity. A somewhat sim- 

 ilar process of reasoning led, in 1876, to the 

 proposal of the "plastidules" of Haeekel; 

 in 1889, to the "pangenes" of de Vries and 

 the "plasomes" of Wiesner; in 1892, to 

 the "ideoblasts" of 0. Hertwig and to the 

 "biophores" and "determinants" of Weis- 

 mann. These units were postulated as 

 logically necessary, but had little other 

 basis. 



A great advance was made in 1910 and 

 later when Morgan and his followers fur- 

 nished experimental evidence of the exis- 

 tence of self -perpetuating "genes" ar- 

 ranged in linear order in the chromosomes, 

 and in 1934 when Painter and in 1935 when 

 Bridges furnished visible evidence of tlieir 

 locations in the giant chromosomes of the 

 salivary gland cells of Diptera. Whether 

 or not these are units of heredity, as many 

 have held, there are certainly differential 

 factors in these loci which appear to have 

 the properties of panmerism, that is, indi- 

 vidual assimilation, growth and division, 



