FOEMATION OF CELLS— TISSUES 



307 



Fig. 663. 



such a way that half of each origkial fibre points its base to one 

 centrosphere or pole, and the other half to the other. The two 

 sets of fibrils so formed then separate and travel back along the 

 spindle fibres towards the two poles of the nucleus, changing 

 their positions as they go till their convex sides point towards 

 them. They thus collect into two places which are determined 

 by the position of the poles of the nucleus, or centrospheres if 

 the latter are present, and present there the appearance of two 

 somewhat star-shaped aggregations. This is known as the 

 diaster stage. The chromosomes at each pole now become 

 united by their ends and constitute two new nuclei, each gradu- 

 ally becoming well defined by the appearance of a nuclear 

 membrane, and the original appearance is regained, nucleoli 

 appearing in each new nucleus , The spindle fibres may remain 

 connecting the new nuclei for a time, as is the case when a cell- 

 wall is formed immediately ; or they may disappear at once. 

 In the cases in which a centrosphere has been observed it 

 divides at this stage, and the two new centrospheres may be 

 distinguished in contact with some point of the new nucleus. 



In ordinary cell-division the formation of the new nuclei is 

 followed by the appear- 

 ance of a oeU-waU. in the 

 original position of the 

 equatorial plate. If the 

 spindle fibres do not 

 stretch completely across 

 the ceD, others are formed 

 beside the original ones, 

 till the spindle is in con- 

 tact with the lateral cell- 

 walls. Granules which 

 have been floating in the 

 cell-protoplasm are to be 

 seen streaming along the 

 spindle fibres till they 

 form a plate stretching 

 across the middle of the 

 cell from wall to wall. 

 From this ceU-plate the septum of cellulose is formed. The 

 central portion of the plate may be formed before the extension 

 of the spindle across the cell is complete. 



A variation of this process is seen in the formation of the 

 spores of certain plants ; for instance, the microspores of Dicotyle- 



FiG. 662. 



So© 



Fig. 662. Yeast plant in process 



of development. Fiy. 663. 



ConferDaylomeratii.s\iO\\\n%t\ie 

 progressive stages of gemma- 

 tion or budding (&, c,d,e). a. 

 Terminal cell. After Von Molil. 



