Bigelow. — Nuclear Cycle of Gonionemus murbachii. 



PLATE 1. 



All figures are from sections. 



Fig. 1. Section of female gonad crosswise to radial canal, showing the general 

 arrangement of the developing germ cells. X 500. 



Fig. 2. Section of male gonad crosswise to radial canal, to show general arrange- 

 ment of the various cell generations. X 500. 



Fig. 3. " Resting" stage of endodermal cell. A part only of the cytosome is shown. 

 The nucleus is in the reticulate condition. X 2600. 



Fig. 4. Early prophase of endodermal cell. The karyosomes have hecome promi- 

 nent, and the karyoplasm less dense. X 2600. 



Fig. 5. Later prophase. The karyosomes are larger and the nucleolar shell has 

 commenced to break down. X 2000. 



Fig. 6. Still later prophase of endodermal cell. The karyosomes have become 

 condensed into definite segments. X 2600. 



Fig. 7. Part of a crushed nucleus in prophase, showing the breaking down of the 

 nucleolar shell. The plasmosome is vacuolate ; only a few of the karyo- 

 somes are shown. X 2600. 



Fig. 8. Late prophase. The nuclear membrane has disappeared. Several chro- 

 mosomes have been formed. X 2000. 



Figs. 9, 10. Metaphase, seen in polar view. There are about 24 chromosomes in 

 each nucleus, many of them dumb-bell-shaped. X 2000. 



Fig. 11. Side view of metaphase of endodermal cell. The centrosomes are now 

 visible. Several chromosomes have commenced individually their 

 migration toward the poles. X 2600. 



