MITOSIS IN NOCTILUCA MILIARIS AND ITS BEAR- 

 ING ON THE NUCLEAR RELATIONS OF 

 THE PROTOZOA AND METAZOA. 



GARY N. CALKINS. 



TABLE OF CONTENTS. 



PAGE 



I. Material and Methods 715 



II. Observations 716 



A. The Resting Nucleus 716 



B. The Sphere in the Resting Cell 717 



C. The Nucleus during Division 718 



1. The Chromatin 719 



a. Prophase 719 



b. Metaphase 722 



c. Anaphase 722 



d. Telophase 723 



2. The Nuclear Membrane 725 



3. The Sphere 725 



4. The Mantle-fibers 729 



5. The Centrosome 731 



D. The Mechanism of Mitosis in Noctiluca 737 



III. The Nuclear Relations of Noctiluca to Metazoa and Protozoa 738 



A. Relations to Metazoa 738 



B. Relations to Protozoa 739 



1. Origin of Chromosomes 739 



2. Origin of Centrosome and Sphere 744 



IV. Summary of Observations 757 



In view of the probable origin of the Metazoa from the 

 colony-forming Protozoa, many observers have attempted, more 

 or less definitely, to trace the phylogenetic development of 

 special parts of metazoan cells from analogous parts in Protozoa. 

 Prominent among such attempts have been those of Biitschli 

 ('9l), Heidenhain ('94), Hertwig ('96), and Lauterborn ('96), and 

 the two last-named writers have shown that, from the least 

 differentiated parts of the most primitive protozoan nuclei, a 

 progressive series may be traced which culminates in Nocti- 



