CELLS. 



29 



Fig. 9. 



Whether division ever take place in other cells than these 

 is not yet determined. If it be allowable to explain, by this 

 process of division, the occurrence 

 of constricted cells with two nuclei, 

 we may suppose it to take place 

 in the nerve-cells, which, in young ' 

 mammalia, are not unfrequently 

 more or less divided or even united 

 merely by a narrow isthmus, and 

 also in the ciliated epithelium cells, 

 which, although rarely, present two 

 or three enlargements lying one 

 behind the other, each with a nu- 

 cleus. A peculiar kind of cell- 

 development, which is very closely 

 related to division, occurs in the 

 formative cells of the ivory, which 

 as they go on growing, multiply their nuclei and become con- 

 stricted from time to time, so that whilst the portion next 

 the ivory ossifies, the other serves in a manner as a reserve 

 for the subsequent formation of fresh ossifying tissue. 



[Schwann knew nothing of the occurrence of cell-division. 

 The first who observed it in the blood-corpuscles of embryos 

 was Remak ('Med. Verein/ 1841, No. 27; Schmidt, ' Janr- 

 biicher/ 1841, p. 145; Canstatt, 'Jahresb./ 1841), yet he 

 subsequently retracted his opinion ('Diagn. und Pathol. 

 Untersuchungen/ p. 100), and only now, since I have con- 

 firmed it and declared it to be true (Wiegm. 'Archiv/ Jahrg. 13, 

 Bd. 1, p. 19), has he again advocated it ( f Entwick. d. Wirbel- 

 thiere/ I). It is extremely probable that this mode of cell- 

 development occurs very extensively, and it may perhaps turn 

 out that in many embryos and adult tissues, in which a self- 

 multiplication of the cells is certain, and yet in which no 

 parent cells with secondary cells can be demonstrated, cell- 

 development by division may occur instead of endogenous 

 Fig. 9. Dentine cells from the dog, x 350. 



Enstehung Thierischen Zellen,' &c., Mull.Archiv, 1852), which by no means deserves 

 the epithet of " no longer available ;" in fact, Remak's views seem to be essentially 

 correct. — Eds.] 



