CELLS. 



55 



Fig. 9. 



nucleus, apparently by division ; the cells then suffer constriction in 

 the middle, and contract more and more around the nuclei, as they recede 

 from each other, and at last separate into two cells, each of which con- 

 tains a nucleus. In the chick we see blood-corpuscles in all conceiva- 

 ble stages of separation, so that at length they are connected only by 

 a delicate thread, and there can be no doubt whatever as to the actual 

 occurrence of this process. 



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, al- 

 though rarely, present two or three 

 enlargements lying one behind the 

 other, each with a nucleus. 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 constricted 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, " Jahrbiicher," 1841, p. 145; Can- 

 statt, "Jahresb.," 1841), yet he subsequently retracted his opinion 

 (" Diagn. und Pathol. Untersuchungen," p. 100), and only now, since I 

 have confirmed it and declared it to be true (Wiegm. " Archiv," 

 Jahrg. 13, Bd. 1, p. 19), has he again advocated it ("Entwick. d. Wir- 

 belthiere,' I.) It is extremely probable that this mode of cell-development 

 occurs very extensively, and it may perhaps turn out that in many em- 

 bryos and adult tissues, in which a self-multiplication of the cell is cer- 

 tain, and yet in which no parent cells with secondary cells can be demon- 

 strated, cell-development by division may occur instead of endogenous 

 cell-development. It is certain that the transverse and longitudinal 

 division of the Protozoa is to be placed here, since these animals have 



FIG. 9. Dentine cells from the dog ; magnified 350 diam. 



