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INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



way, but no cell plate is laid down. The two daughter nuclei, 

 which are diploid, remain inside the cell (Fig. 27 A-F). 



2. By a "sticky" type of division. Here the chromosomes behave 

 normally up to the early anaphase stage. After this, one or more of 

 them fail to separate, forming chromosome bridges which persist 

 during the telophase as well as the resting stage. As a result a 

 single dumbbell-shaped tetraploid nucleus is formed whose middle 

 portion may be broad or narrow depending on the number of 

 chromosome bridges present (Fig. 27 G-K). 



3. By endomitosis. 6 Here the nucleolus and the nuclear mem- 

 brane remain intact and there is no spindle formation. The chro- 



Fig. 28. Diagrams showing "endomitosis" in tapetal cells of Spinacia oleracea. 

 A, endoprophase. B, endometaphase. C, endo-anaphase. D, endotelo phase. 

 (Drawing supplied by Dr. E. R. Witkus.) 



mosomes contract and split longitudinally, but all of them remain 

 within the same nucleus, which becomes tetraploid (Fig. 28). 



The first nuclear division in a tapetal cell is often followed by 

 further divisions. Some of the divisions may be accompanied by 

 nuclear fusions, resulting in one or more large polyploid nuclei. 

 The latter may, however, divide again and give rise to smaller nu- 

 clei. Since this type of behavior is very frequent in tapetal cells, 

 it is unnecessary to give specific instances. 



An interesting condition has been reported in certain haploid and 



6 This type of division was first postulated by Meyer (1925). In the tapetal 

 cells of Leontodon he found diploid nuclei in younger stages and polyploid nuclei in 

 older stages. Since no spindle fibers were observed, he concluded that there was an 

 "internal division" of the chromosomes without any nuclear division. See also 

 Brown (1949) who has recently given a detailed account of endomitosis in the 

 tapetal cells of tomato. 



