20 



NORMAL HISTOLOGY. 



during the resting stage is single, but its multiplication early takes 

 place in the dividing nucleus and anticipates the establishment of the 

 poles of the new nuclei; the apices of the nuclear spindles coincide 

 with the attraction-spheres, which are, probably, potent factors in 

 determining the exact position of the spindles and, consequently, the 

 plane of division. 



Fission of the nucleus is ordinarily followed by cleavage of the 

 protoplasm, the resulting new cells being entirely distinct elements. 



A deviation from this usual 

 procedure is, however, some- 

 times encountered where the 

 division of the nucleus has 

 not been followed by cleavage 



FIG. 13. 



Segmenting ova of ascaris megalocephala : A , cell Large marrow-cell : the nu- 



contains nucleus, two centrosomes (c), surrounded by cleus has undergone repeated 



attraction-spheres, and adherent polar body (p) ; B, division without cleavage of the 



beginning polar striation around the centrosomes and protoplasm, 



attraction-spheres; C, cell viewed from polar field, the 

 striation proceeding from the centrosome ; D t cell seen 

 from the side, apices of nuclear spindle correspond with 

 centrosomes. (After Boveri.) 



of the cell-protoplasm, the latter remaining undivided even after the 

 repeated division of the nuclei. Examples of such ' ' endogenous' ' 

 formation are seen in the multinucleated giant marrow-cells. 



These complicated phenomena can be satisfactorily observed only 

 in suitable preparations and with adequate optical appliances; the 

 dividing-cells of the surface epithelium of very young larval newts 

 (ten to twenty millimetres long) supply admirable views of all stages 

 of karyokinesis. In order to obtain permanent preparations, how- 

 ever, these transient changes must be" fixed" by powerful reagents, 

 insuring the instantaneous death of the tissue (see Appendix); 

 otherwise the cycle, which occupies only from two to three hours, 

 and often even less time, will have been completed, and all trace of 

 the figures lost. The command of at least five hundred diameters, 

 with unexceptionable definition, is likewise essential for the careful 

 study of these changes. While most favorably seen in fixed and 

 stained preparations, the karyokinetic figures may be observed in 

 living cells, thus proving that they in no wise depend upon reagents 

 for their existence. 



