HISTORICAL SKETCH 439 



cells. It was now possible to describe the life cycle in terms of cell 

 successions; and since the egg was seen to be a descendant of the egg of the 

 previous generation, it became evident that there has been an uninter- 

 rupted series of cell-divisions from the remote past down to the organisms 

 in existence today. The statement of this conception, without, however, 

 any necessary emphasis on cells rather than protoplasm itself, is known 

 as the law of genetic continuity. In the words of Locy (1915): 



The conception that there is unbroken continuity of germinal substance 

 between all living organisms, and that the egg and the sperm are endowed with 

 an inherited organization of great complexity, has become the basis for all current 

 theories of heredity and development. So much is involved in this conception 

 that ... it has been designated (Whitman) "the central fact of modern biol- 

 ogy." The first clear expression of it is found in Virchow's Cellular Pathology, 

 published in 1858. It was not, however, until the period of Balfour, and through 

 the work of Fol, Van Beneden (chromosomes, 1883) Boveri, Hertwig, and others, 

 that the great importance of this conception began to be appreciated, and came 

 to be woven into the fundamental ideas of development. 



Mitosis and Meiosis. — We have pointed out that cells were once 

 believed to arise de novo from a mother liquor, or "cytoblastema," This 

 misconception was removed by later investigations in which it was shown 

 beyond question that cells arise in general by the division of preexisting 

 cells. By several early observers the nucleus was seen to have a more or 

 less prominent part in the process, its division preceding that of the cell; 

 but, in the words of Wilson (1900), 



... it was not until 1873 that the way was opened for a better understanding 

 of the matter. In this year the discoveries of Anton Schneider, quickly followed 

 by others in the same direction by Biitschli, Fol, Strasburger, Van Beneden, 

 Flemming, and Hertwig, showed cell-division to be a far more elaborate process 

 than had been supposed, and to involve a complicated transformation of the 

 nucleus to which Schleicher (1878) afterward gave the name karyokinesis. It 

 soon appeared, however, that this mode of division was not of universal occur- 

 rence; and that cell-division is of two widely different types, which Van Beneden 

 (1876) distinguished && fragmentation, corresponding nearly to the simple process 

 described by Remak, and division, involving the more complicated process of 

 karyokinesis. Three years later Flemming (1879) proposed to substitute for 

 these terms direct and indirect division, which are still used. Still later (1882a) 

 the same author suggested the terms mitosis (indirect or karyokinetic division) 

 and amitosis (direct or akinetic division), which have rapidly made their way 

 into general use, though the earlier terms are often employed. Modern research 

 has demonstrated the fact that amitosis, or direct division, regarded by Remak 

 and his followers as of universal occurrence, is in reality a rare and exceptional 

 process; ... it is certain that in all the higher and in many of the lower forms 

 of life, indirect division or mitosis is the typical mode of cell-division. 



The chromosomes appeared in published figures long before their 

 significance was appreciated. Thus in 1848 Wilhelm Hofmeister exam- 



