Plate XXI 



Four stages of the segmentation of the egg of Ascaris megalocephala univalens, 

 somewhat diagrammatic, after BOVERI (1903 a). 



a. Close of first division ; in the upper cell the long chromosomes are breaking 

 up into short units and throwing off the thickened ends. 



b. Anaphase of second division ; in the upper cell the thickened ends of the 

 chromosomes take no part in the mitosis. 



Four-cell stage with resting nuclei. 



Metaphase of third division, in the upper cells in face, in the lower cells 



from the side. The left lower cell is undergoing "chromatin diminution." 



V 



