10 INTRODUCTION TO CYTOLOGY 



cell-formation, and showed that what had been taken for the latter was 

 only a special case of the former. Nageli's conclusions were supported 

 by new evidence furnished by other investigators, who further demon- 

 strated that not only vegetative cells but also those reproductive cells 

 (in thallophytes) which Nageli thought in some cases might be formed 

 freely, originate by a modified process of cell-division. It was now clear 

 that cells arise only from preexisting cells, a conception which had been 

 emphasized by Remak (1841) and which Virchow (1855) expressed in the 

 dictum "omnis cellula e cellula." 



Opinions concerning the origin of the nucleus and its role in cell- 

 division varied greatly among these workers, reliable observations being 

 as yet insufficient to allow the formulation of any definite conclusion. 

 In 1841 Henle believed with Schleiden and Schwann that the nucleus was 

 formed by the aggregation of "elementary granules," and that it was not 

 constantly present. Goodsir looked upon the nucleus as the reproduc- 

 tive organ of the cell. Von Kolliker in 1845 asserted that nuclear divi- 

 sion precedes the division of the cell, and 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 mem- 

 brane were supposed to divide in turn by simple constriction. Just such 

 a process, though evidently very exceptional, has been observed at a 

 more recent date by Conklin (1903). In describing a case of nuclear 

 division Wilhelm Hofmeister (1824-1877) stated that the membrane of 

 the nucleus dissolved, the nuclear material then separating into two 

 masses around which new membranes were formed (1848, 1849). It was 

 generally believed, however, that the origin of nuclei by division was 

 of rare occurrence, and that ordinarily the nucleus dissolved just before 

 cell-division, two new ones forming de novo in the daughter cells. Von 

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

 "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 produc- 

 tion 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. Naegli thinks that the process is 

 quite similar to that in cell-division, the membrane of the nucleus form- 

 ing a partition, and the two portions separating in the form of two dis- 

 tinct cells." 



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

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

 division became known in detail, and its probable significance pointed 

 out. 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 'onrnis nucleus e nucleo" (1882). (See footnote, p. 143.) 



