64 



CELL-DIVISION 



body and membrane (Fig. 24). 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 simple 

 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. 24,/). It was not until 1873 th at the way was opened 

 for a better understanding of the matter. In this year the discoveries 

 of AntonSchneidfij, 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 

 (^y^) afterward gave the name 

 of karyokincsis. It soon ap- 

 peared, however, that this mode 

 of division was not of universal 

 occurrence ; and that cell-divi- 

 sion is of two widely different 

 types, which Van Beneden ('76) 

 distinguished as fragmentation, 

 corresponding nearly to the 

 simple process described by 

 Remak, and division, involving 

 the more complicated 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 amitosis (direct or akinetic division), which have rapidly made 

 their way into general use, though the earlier terms are often em- 

 ployed. 



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 and in the cells of transitory embry- 

 onic envelopes, where it is of frequent occurrence. Whether this 



Fig. 24. — Direct division of blood-cells in 

 the embryo chick, illustrating Remak's scheme. 

 [Remak.] 



a-e. Successive stages of division ; f. cell 

 dividing by mitosis. 



