THE DEVELOPMENT OF DUMONTIA 279 



very heavily. In a few cases the chromatin appears to be in the 

 form of two spherical bodies of almost the same size (fig. 2, M). 

 A small dark staining body is always present at each pole of this 

 spindle-shaped nucleus. These bodies are evidently similar to 

 the eentrosomes occurring in the tetrasporangia of Polysiphonia 

 (Yamanouchi) and to the few eentrosomes seen in Delesseria 

 (Svedelius). In both these forms, the eentrosomes apparently 

 arise de novo and no radiations have been seen running out from 

 one of these bodies into the cytoplasm. Division has occurred 

 in the next stage which was observed and the daughter nuclei 

 are just separating (fig. 2, M). The chromatin in each of these 

 nuclei appears to be in the form of a disc which is composed of 

 four granules. No nuclear membrane is present around either 

 of these chi'omatin masses. The chromatin is on structures 

 which resemble spindle fibres. The cytoplasmic jacket which 

 surrounded the mother nucleus has not begun to divide. Each 

 of the two daughter nuclei moves to an end of the cell before 

 it begins to divide (fig. 2, N). The chromatin in the nuclei 

 at this stage may be in one, two or four small round bodies. 

 The diameter of these nuclei is 4.4/x. 



At a little later stage the cleavage furrow which wdll di\dde 

 the tetrasporangium into two cells is just being formed (fig. 2, 0). 

 The first cleavage furrow as seen in a cross-section of the thallus 

 is perpendicular to the longitudinal axis of the tetrasporangium, 

 that is, parallel to the surface of the thallus. The cytoplasm 

 of this stage is considerably vacuolated and for the most part 

 peripheral. The chromatophores have apparently decreased 

 in number and they do not stain as readily as they did in the 



matophores are distributed on cytoplasmic net and a number of small chro- 

 matin granules are present. X 700. Q. Longitudinal section of a two-celled 

 tetrasporangium, nuclei preparing for division. X 700. R. Similar section of 

 a four-celled tetrasporangium, showing the jacket of sterile hyphae and the 

 contents of a young tetraspore. X 700. S. Similar section of a four-celled 

 tetrasporangium; the quadrant walls are perpendicular to each other. X 700. 

 T. Longitudinal section of a mature tetrasDore, showing the arrangement of 

 the chromatophores. X 700. U. Surface view of chromatophores of a mature 

 tetraspore, showing perforations in their walls. 



