86 AQUATIC PHYCOMYCETES 



called "chromidial" net is cytoplasmic or nuclear is not known. It 

 first appears during the later stages in zoospore formation in the spo- 

 rangium, is present in the young thallus, and passes, finally, along 

 with the nucleus, into the new sporangium, where it becomes dispersed 

 in the cytoplasm. During development chromatin is constantly extruded 

 from the nucleus. After migrating from the mature prosporangium into 

 the enlarging sporangium (Fig. 5 C) the nucleus divides mitotically. 

 The daughter nuclei then continue to divide simultaneously (Fig. 5 

 D-E) until, in large sporangia, several hundred have been formed. 

 In the mitotic process an intranuclear spindle is formed, on which at 

 the metaphase from ten to twelve minute chromosomes can be observed. 

 Only a small amount of chromatin is used in the production of the chro- 

 mosomes, the remainder forming a layer on the inner periphery of 

 the nuclear membrane. This layer is visible during the prophases and 

 is ultimately set free in the cytoplasm, where it is used in the growth 

 of the developing sporangium. Soon after the spindle becomes evident 

 the nuclear membrane disappears at the poles, allowing them to pro- 

 trude slightly into the cytoplasm. The remainder of the nuclear mem- 

 brane then contracts. Centrosome-like bodies are visible at the poles 

 of the spindles at this stage. Upon the separation of the daughter nuclei 

 the nuclear membrane vanishes and the peripheral chromatin mass 

 contracts to form a more or less globular body in the cytoplasm. The 

 chromosomes then become aggregated at the periphery of the new 

 daughter nuclei in contact with a light-staining network which forms 

 the bulk of each new nucleus. Delicate strands persist for a time be- 

 tween the daughter nuclei, but finally disappear. 



Nuclear divisions are completed before cytokinesis is initiated. Cleav- 

 age lines separating irregular, uninucleated, polyhedral segments of 

 the cytoplasm then make their appearance. In each of the segments 

 the small oil droplets dispersed in the cytoplasm fuse to form a large 

 globule which is in close contact with the nucleus (Fig. 5 F, p. 80). 

 The deep-staining chromidial mass then appears around both nucleus 

 and globule. At this stage the cleavage planes disappear for a time, to 

 reappear again shortly before discharge. The nucleus of the zoospore 

 is almost centrally located, in close proximity to the globule; the sur- 

 rounding chromidial mass extends to the point of attachment of the 



