432 INTRODUCTION TO CYTOLOGY 



clearly observed the poleward migration of what we now call chromo- 

 somes^ in fresh microsporocytes of Tradescantia (Fig. 230). Remak, as a 

 result of his observations on blood cells in the chick embryo, formulated a 

 definite theory of cell-division (1841, 1858). He believed cell-division to 

 be a "centrifugal" process: the nucleolus, nucleus, cytoplasm, and cell 

 membrane were supposed to divide in turn by simple constriction. Von 

 Mohl, who in the main agreed with Hofmeister, wrote as follows (1851) : 



The second mode of origin of a nucleus, by division of a nucleus already 

 existing in the parent-cell, seems to be much rarer than the new production of 

 them. . . . [And again] ... it is possible that this process [nuclear division] 

 prevails very widely, since ... we know very little yet respecting the origin of 

 nuclei. Nageli thinks that the process is quite similar to that in cell-division, 

 the membrane of the nucleus forming a partition, and the two portions separating 

 in the form of two distinct cells. 



It was not until many years later, in connection with researches upon 

 syngamy and embryogeny, that the behavior of the nucleus in cell- 

 division became known in detail and its probable significance pointed out 

 (p. 441). In 1879 Eduard Strasburger (1844-1912) announced definitely 

 that nuclei arise only from 'preexisting nuclei. W. Flemming was led to 

 the same conclusion by his studies on animal cells and expressed it in the 

 dictum, omnis nucleus e nucleo (1882a). 



The Protoplasm Doctrine. — The cell theory and all of its corollaries 

 were placed in a new light with the development of a more adequate 

 conception of the significance of protoplasm. A number of early investi- 

 gators had seen protoplasm and had been impressed by certain of its 

 activities. As early as 1772 Corti, and a few years later Fontana (1781), 

 saw the rotation of the "sap" in the Characefe and other plants. After 

 being long forgotten, this phenomenon was rediscovered by L. C. 

 Treviranus (1811) and G. B. Amici (1819). The cell to its discoverers 

 meant nothing more than a wall surrounding a cavity; they spoke only in 

 the vaguest terms of the "juices" present in cellular structures. The 

 founders of the cell theory held a position but little in advance of this; 

 they observed the cell contents but regarded them as of relatively slight 

 importance. 



Felix Dujardin (1801-1860) in 1835 described the "sarcode" of the 

 lower animals as a substance having the properties of life. Von IVIohl had 



' Concerning the behavior of these bodies in the formation of daughter nuclei, 

 Hofmeister says: "Ich kann die Bildung der Tochterkerne in keiner andern Weise 

 mir denken, als dass die im Centrum der Mutterzelle angesammelten eyweissartigen 

 Stoffe, aus zur Zeit unbekannten Ursachen in zwei Gruppen sich sondern, die in Form 

 abgeplatteter elliptischer Spharoide sich individualisieren; spater nach Aussen sich 

 mit einer Membran bekleiden und im Innern Bildungen aus dichteren Stoffen erzeugen 

 konnen." Similar statements accompany his figures of Psilotum in Die Lehre von 

 der Pfianzenzelle (1867). 



