No. 3.] THE PROTOZOA AND METAZOA. 739 



and after division again become disintegrated ; (7) as in the 

 Metazoa the chromosomes divide by longitudinal division ; (8) 

 the centrosomes, finally, are equivalent ; they are the focal 

 points of the mantle-fibers ; they lie in the spheres during 

 activity, and they divide during the anaphase in preparation for 

 the ensuing division. There is some evidence that, as in Ascaris 

 viegalocephala nnivalens (Brauer), they come from the nucleus. 

 In only one respect does the mitotic-figure in Noctiluca differ 

 from that of the Metazoa — the nuclear membrane does not 

 entirely disappear. In all other respects its description would 

 answer for that of any ordinary metazoan mitotic-figure. 



B. Relations to Protozoa. 



Mitosis in Noctiluca is so similar to that in Metazoa that in 

 itself it throws little light on the origin of the process. The 

 rapid increase of our knowledge of indirect division in the 

 Protozoa has, however, made it possible to draw an accurate 

 comparison between Noctiluca and other Protozoa in which the 

 phenomena of mitosis appear in a still simpler form ; and here 

 we find some light on the possible origin of mitosis. 



I . Origin of Chromosomes. 



In many primitive forms of protozoan nuclei, the chromatin 

 is compressed into a single homogeneous sphere {Uroglena, 

 Dinobryon, Eiidorina, etc.), and with no indication of "achro- 

 matic" substances in the form of "ground-substances" or 

 "nuclear-sap." In other forms of nuclei closely allied to 

 these, the chromatin in the resting stage is similarly collected 

 into a homogeneous sphere, but it lies imbedded in the nuclear 

 ground-substance, the whole surrounded by a nuclear mem- 

 brane {Actinophrys sol, Gruber ('83), Heterophrys, Acanthocystis, 

 Artodiscus, etc., Penard ('89), and many Flagellates). Penard 

 and Gruber found that in those cases where the chromatin 

 forms a single mass it becomes divided into two, three, or 

 more separate portions. Rhumbler ('90) agrees with Gruber 

 and Penard in such an arrangement of the chromatin in 

 Rhizopods and Heliozoa, and Wolters ('91), Labbe ('97), Clarke 



