90 AQUATIC PHYCOMYCETES 



centered on two disc-shaped bodies lying on the nuclear membrane. 

 In the metaphase the chromosomes form a crowded ring around the 

 periphery of the equator, the nucleolus persisting as a crescent-shaped 

 dark-staining body at the side of the spindle (Fig. 6 E, p. 88). Actual 

 separation of the individual chromosomes has not been observed, but 

 in the early anaphase the groups of daughter chromosomes form two 

 somewhat flattened loops. Later these turn into more flattened cres- 

 centic bands at the poles. The spindle then elongates and penetrates 

 the nuclear membrane, which disappears except at the equatorial re- 

 gion, where it persists for a time. The nucleolus is now liberated into 

 the cytoplasm, where it may remain for a long period before disinte- 

 grating and being absorbed. Definite polar radiations appear in the 

 cytoplasm around the chromosome groups. Evidence at present in- 

 dicates that in the telophase each of the chromosome masses some- 

 how becomes surrounded by a membrane. From each nucleolus-like 

 body the reticulum of the new nucleus then takes its origin. There is 

 nothing to show that the chromosome mass fragments into successively 

 smaller and ultimately dispersed bodies. It is believed that the nucleolus 

 of the new nucleus consists of the residue of the chromosome mass 

 remaining after the reticulum is formed. 



Cytokinesis, which occurs after division of the nucleus, appears to 

 be independent of the activity of the achromatic spindle. Actual wall 

 formation in the spindle organs (turbinate cells) is initiated by a periph- 

 eral furrow, which progresses centripetally in the equatorial region. 

 A thin membrane, continuous with the cell wall, extends inward. 



The rudiment of the sporangium is uninucleate (Fig. 6 E), the nu- 

 cleus being surrounded (as in Polyphagia) by deep-staining material. 

 The number of nuclei is increased by repeated simultaneous mitotic 

 divisions like those occurring in the spindle organs, and, coincidently, 

 the size of the individual nuclei decreases (Fig. 6 F). The spore origins 

 are cut out by a process of progressive cleavage around the individual 

 nuclei (Fig. 6 G-I). No cytological observations on the nuclear be- 

 havior in proliferated sporangia are recorded. The thick-walled a- 

 sexually formed resting spores (Fig. 6 J) are predominantly uninu- 

 cleate, although occasional binucleate ones occur. 



A cytological investigation by Hillegas (1940) of the monocentric 



