338 WOOLFORD B. BAKER. 



nomena during mitosis and yet not until comparatively recent 

 time were the nuclear phenomena here critically described and 

 figured. Much controversy has arisen because of superficial 

 observation as regards both the structure of the resting nucleus 

 and the process of nuclear division. Tschenzoff (1916), presented 

 one of the most complete and critical studies of Euglena viridis 

 recorded and attempted to interpret the nuclear phenomena. 

 Belar likewise, gave a good account of the process in Astasia, 

 although his descriptions and figures seem to be somewhat 

 incomplete. Other workers, Rhodes and Kirby (MSS.) on 

 Heteronema, Hall (1923) on Menoidium have given more detailed 

 accounts. Much further study must be made before many of the 

 disputed points can be settled. 



The Resting Nucleus. 



The nuclei of flagellates present varied appearances. Prowazek 

 (1903), attempted to classify the various types and Dobell (1908) 

 grouped these types under the following four classes, adding a 

 fifth class himself: 



(1) Simple nuclei, with an evenly distributed chromatic network, 



and no internal structures (karyosomes, division centers, 

 etc.), e.g., Ilerpetomonas. 



(2) Vesicular nuclei, with direct division; with a central chro- 



matin mass surrounded by a clear zone, across which a 

 more or less distinct network extends outward to the 

 nuclear membrane. Such a nucleus may be seen in some 

 species of Bodo, and is well represented by Copromonas. 



(3) Centronuclei containing a "nucleolo-centrosome" (Keuten, 



1895) and separate chromatin masses. This type of 

 nucleus is characteristic of Euglena and its allies. (The 

 centronucleus, as defined by Boveri, is a nucleus which 

 contains a cyto-center, either in consolidated or diffuse 

 form. In the case of Euglena, etc., the cyto-center is the 

 nucleolo-centrosome, i.e., is of the consolidated type.) 



(4) Vesicular nuclei, with karyokinetic division, e.g., Polytoma, 



Chlamydomonas , etc. 



( 5) Nuclei in which the achromatic division center lies freely in 



the cell, whilst the chromatin is diffuse in the form of 

 chromidia. 



