﻿272 BOTANICAL GAZETTE [april 



the nucleus and in the quantity of chromatin substance. In the 

 higher plants, it is during this division that the reduced number 

 of chromosomes appears, 



In the homotypic division, the chromatin spirem or thread 

 does not undergo a longitudinal fission. During the develop- 

 ment of the spindle the granddaughter chromosomes are arranged 

 in pairs in the nuclear plate by means of the spindle fibers. The 

 segments of each pair often lie closely applied side by side in 

 such a way as to suggest that the daughter spirem had split 

 longitudinally. This arrangement of the segments with refer- 

 ence to each other does not ahvays obtain, for they may fre- 

 quently diverge, one lying tangentially upon one side of the 

 equator and the other upon the other side. In the pollen 

 mother-cell of Lilium and in certain other plants, the homotypic 

 may follow the heterotypic division without the intervention of 

 a complete resting stage, but even here there is a greater or less 

 tendency for the daughter nucleus to pass into the structure of 

 the resting condition. In all cases, however, the identity of the 

 individual chromosomes is lost from observation in the daughter 

 nucleus. 



THE HOMOLOGY OF THE MICRO- AND MACROSPORE MOTHER-CELLS. 



L 



From the foregoing statements and in the light of the recent 

 researches of Juel (iQOo) and Schniewind-Thies (1900), it seems 

 that all cytological evidence indicates clearly and unmistakably 

 that the pollen mother-cell and embryo-sac mother-cell are 

 homologous. 



The writer has already shown that the first nuclear division in 

 the macrospore mother-cell of Helleborus is heterotypic, and 

 the same is true for Podophyllum. In the development of the 

 embryo-sac of Helleborus, which may be regarded as typical for 

 dicotyledonous plants, as well as for many monocotyledonous 

 species, the heterotypic and homotypic mitoses are ea 

 followed by cell-divisions, giving rise to four potential macro- 

 spores, homologous with the four granddaughter cells in the 

 pollen mother-cell. The fact that occasionally in Helleborus 

 cell-division does not take place until after the second mitosis 



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