

CURRENT LITERATURE 







NOTES FOR STUDENTS 



Mitosis in Osmunda. — Cytologists are familiar with the two outstanding 

 views, associated respectively with the names of Gregoire and Farmer, 

 regarding the method of chromosome reduction. According to the first view 

 the doubleness of the spirem of the early heterotypic prophase, unlike that 

 of the somatic prophase, is due to a lateral conjugation of threads representing 

 entire chromosomes to form bivalents which are separated at the heterotypic 

 mitosis, a new split functioning in the homotypic. According to the second 

 view the doubleness is due to a split as in somatic mitosis; bivalents are 

 formed by a conjugation of segments of this double spirem which separate 



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in the first mitosis, while the original split functions in the second. A. very 

 complete statement of this latter interpretation has been given by Miss Digby 1 

 in a new account of mitosis in Osmunda. 



In all the archesporial divisions, including the last, the chromosomes 

 undergo a longitudinal splitting during early telophase. The homogeneous 

 daughter threads become beaded as the split between them widens, and with 

 many small connecting strands eventually form a faint resting reticulum which 

 bears many small granules, and in which the limits of the individual chromo- 

 somes are indistinguishable. Most of the chromatin is collected in three or 

 more nucleoli. In the succeeding prophase the reticulum resolves itself into 

 a number of thin beaded linin threads; these run in parallel pairs and are 

 regarded as the two reassociating halves of the chromosomes split in the 

 preceding telophase. As the association becomes closer, the material of the 

 threads is progressively concentrated, until it takes the form of a double 

 spirem which segments into split chromosomes. These are separated into 

 their component halves at anaphase and undergo a new splitting during 

 telophase. Nuclei may go from the telophase of the last premeiotic division 

 directly into the heterotypic prophase, or may pass through an intervening , 

 resting stage. 



In the heterotypic prophase the reticulum gives rise to beaded "threads" v 

 which become more uniform spirems with a distinct parallelism, just as in the . 

 archesporial prophases. At this stage occurs synizesis, during which the 

 reassociation of the parallel threads to form "filaments" is completed. From 

 the contraction emerges a thick double spirem homologous with the double 



1 Digby, L., On the archesporial and meiotic phases of Osmunda. Ann. Botany 



33:135-172. pis. 8-12. fig. 1. 1919. 



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