46 



CELL-DIVISION 



with the division of the nucleolus, is continued by simple constriction 

 and division of the nucleus, and is completed by division of the cell- 

 body and membrane (Fig. i8). For many years this account was 

 accepted, and no essential advance beyond Remak's scheme was 

 made for nearly twenty years. A number of isolated observations 

 were, however, from time to time made, even at a very early period, 

 which seemed to show that cell-division was by no means so sim- 

 ple an operation as Remak believed. In some cases the nucleus 

 seemed to disappear entirely before cell-division (the germinal vesicle 

 of the ovum, according to Reichert, Von Baer, Robin, etc.); in others 

 to become lobed or star-shaped, as described by Virchow and by 

 Remak himself (Fig. i8,/). 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 com- 

 plicated transformation of the 

 nucleus to which Schleicher 

 / ('78) afterwards gave the name 

 of Karyokiiiesis. It soon ap- 

 peared, however, that this mode 



d ' € 



Fig. 18. — Direct division of blood-cells in 



the embryo cluck, illustrating Remak's scheme. 

 [Remak.] 



a-e. Successive stages of division; / Cell of division waS not of UniVCr- 



dividing by mitosis. g^l occurrcncc ; and that cell- 



division is of two widely different types, which Van Beneden ('76) 

 distinguished as fragvientatwn, corresponding nearly to the simple 

 process described by Remak, and division, involving the more com- 

 plicated process of karyokinesis. Three years later Flemming ('79) 

 proposed to substitute for these the terms direct and indirect division, 

 which are still used. Still later ('82) the same author suggested the 

 terms mitosis (indirect or karyokinetic division) and aviitosis (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 immediate followers as of uni- 

 versal occurrence, is in reality a rare and exceptional process ; and 

 there is reason to believe, furthermore, that it is especially char- 

 acteristic of highly specialized cells incapable of long-continued 

 multiplication or such as are in the early stages of degeneration, 

 for instance, in glandular epithelia, in the cells of transitory em- 



