8 



INTRODUCTION 



completely divided ; and the daughter-cells are themselves capable of 

 karyokinetic division, and so on until the cells thus produced lose this 

 power and die of old age. 



It is possible for a single observer to watch the process of karyo- 

 kinesis through all its phases, which is found to occupy half an hour to 

 three hours ; but the different stages are more easily studied in hard- 

 ened and stained preparations. 



Figure 5 is a reproduction, reduced one-half, of plates selected from 

 the superb Atlas of tJie Fertilization and Karyokinesis of the Ovum, by 

 Professor Edmund B. Wilson, of Columbia University. These are faith- 

 ful reproductions of photographs of the actual processes in the naked 

 egg of the sea-urchin (Toxopneustes variegatus), all magnified in the 

 original 1000 diameters except C, which is magnified 3000 diameters. 



A, x 500, is the ovarian egg, before maturation, with the germinal 

 vesicle (nucleus) and the germinal spot (nucleolus); B, x 500, shows 

 the formation of the asters and the origin of the spindle (one of the late 

 prophases) ; C, x 1500, shows the splitting and separation of the 

 chromosomes (metaphase) ; D, x 500, shows the beginning divergence 

 of the daughter-chromosomes (early anaphase) ; E, x 500, shows com- 

 plete divergence of the daughter-chromosomes (final anaphase) ; F, x 500, 



shows division of the egg nearly 

 completed and re-formation of the 

 daughter-nuclei (telophase). 



Amitosis. Exceptionally, 

 certain cells undergo direct, or 

 amitotic division, there being no 

 formation of chromosomes, spi- 

 rems or spindles, and the centro- 

 somes taking no part in the 

 process. In amitosis the nuclei 

 enlarge and there is a consider- 

 able increase in its chromatin. 

 The nuclei then become con- 

 stricted and finally divide into 

 two, the cell itself afterward fol- 

 lowing this division. 



The physiological significance 

 of amiosis is obscure ; and the 

 process has been the subject of much study and speculation. The chief 

 point of interest, concerning which there is considerable doubt, is the re- 

 lation of the centrosome and the attraction-sphere to this process ; but it 

 is the opinion of most embryologists that the nucleus alone is involved, 



Fig. 6. Groups of cells with amitotically dividing 

 nuclei (Wilson, after Wheeler). 



