30 NORMAL HISTOLOGY. 



tinguish the centrosome, and its constant presence in cells is largely 

 a matter of inference. Sometimes the centrosome is double, the 

 two granules lying close to each other and often being surrounded 

 by a common clear zone of hyaloplasm. 



The first step in the process of cell-division by the indirect method, 

 or karyokinesis, is a division of the centrosome into halves (Fig. 

 15), which separate and pass to opposite points in the cytoplasm. 

 These points are called the poles of the cell, and when the new cen- 

 trosomes reach them they are called the polar bodies. In these situa- 

 tions they are surrounded by a more distinct zone of hyaloplasm 

 than that which enclosed the original parent centrosome, and beyond 

 this the spongioplasm is frequently arranged in radiations of unusu- 

 ally thick fibres. The polar bodies with their clear envelopes and 

 the prominent radiations about them are collectively known as the 

 attraction-spheres (Fig. 8). 



Fig. 8. 



Dividing cell from ovum of ascaris megalocepkahis. (Kostanecki and Siedlecki.) a, polar body, 

 centrosome, surrounded by a clear zone ; b, chromosomes of the dividing nucleus. Be- 

 tween the polar bodies is the achromatic spindle, and radiating from each attraction- 

 sphere are delicate lilaments of spongioplasm. The cytoplasm presents indications of 

 vacuolation. 



While the polar bodies are separating, or after they have passed 

 into the polar regions of the cell, the nucleus begins to show those 

 changes in structure which constitute karyokinesis. This process 

 may be divided into a number of phases, as follows: 



1. The Formation of the Spirem (Fig. 9 ).— This consists in a con- 

 densation of the chromoplasm. The branches of the nuclear fila- 

 ments are withdrawn into the substance of the main fibres, into 

 which the unclear membrane or peripheral network bounding the 



