12 



nucleolus-like mass alone occupies the nuclear cavity, and 

 is probably an extreme phase of synaptic contraction 

 (Fig. 6). This stains deeply, and seems to correspond to 

 the nucleolus-like features presented by the densely 

 concentrated nuclei of a spermatozoid or spirogyra. Some- 

 times the nucleolus becomes vacuolated. 



MITOSIS. 



It will here be convenient to enter into a brief 

 description of the intricate phenomena of nuclear division, 

 and trace the varying phases that the chromatin and other 

 substances pass through in the process. 



There are two forms of mitosis, known respectively 

 as heterotype, dealing with the first or reduction division 

 of a spore mother-cell, i.e., the division which ushers in the 

 gametophyte — sexual forms of the plant, whose nuclei 

 contain only one-half the number of chromosomes found in 

 those of the sporophyte or asexual form ; and the homotype, 

 which takes place in sporogenous cells preceding the 

 mother-cell stages and those (sporogenetic) immediately 

 succeeding to form the spores, also in vegetative or tissue 

 cells generally. Four stages of mitosis are usually 

 described, namely : —Prophase, metaphase, anaphase and 

 telophase. 



HETEROTYPE MITOSIS. 



In order to trace this form of nuclear division, which 

 may conveniently be done in pollen mother-cells, it will be 

 necessary to briefly describe their origin. Certain cells 

 (Fig. 7) lying just beneath the epidermis of a very young 

 anther — hypodermal cells — enlarge as archesporia, i.e., 

 cells marked off for the ultimate production of reproductive 

 cells or spores. They divide into a layer of outer cells, the 

 primary parietal or wall layer of the anther, and an inner 

 primary sporogenous layer, whose cells after several 



