THE MICROSPORANGIUM 43 



making a total of six spindles. However, these spindles have noth- 

 ing to do with the quadripartition of the mother cell, and there is 

 no laying down of centrifugally growing cell plates such as are 

 characteristic of other dividing cells. Instead, constriction furrows 

 now start at the periphery and proceed inward until they meet at 

 the center, so that there is a simultaneous division of the protoplast 

 into four cells, i.e., the microspores. 



In Melilotus alba (Castetter, 1925) vacuoles seem to play a con- 

 spicuous part in cytokinesis (Fig. 33). After Meiosis II, hyaline 

 areas develop between the four nuclei, apparently as the result of 

 a migration of the denser cytoplasm toward the nuclei and an ex- 

 trusion of sap into the regions between them. The small vacuoles 

 arising in this manner soon fuse to form larger ones which virtually 

 split the cytoplasm into four masses. Furrows originating at the 

 surface now grow inward and soon meet the vacuoles. Meanwhile, 

 the mother cell rounds up and secretes a thick layer of callose or some 

 other gelatinous material, which extends inward with the cleavage 

 furrows and eventually completes the division of the cell into the 

 four microspores. 9 



Zea mays (Reeves, 1928) may be taken as an example of the suc- 

 cessive type of microspore formation (Fig. 34). At the end of Meiosis 

 I, thickenings are formed on the spindle fibers at the equatorial 

 region of the cell. They gradually increase in size, coming in con- 

 tact with each other and fusing to form the cell plate. Additional 

 spindle fibers continue to appear just beyond the periphery of the 

 plate so as to increase the diameter of the spindle. At the same time 

 the cell plate extends centrifugally and joins the wall of the mother 

 cell, so as to complete the division of the protoplast into two halves. 

 Now the second meiotic division follows, and a new partition wall 

 develops in each cell in the same way as after Meiosis I, resulting 

 in a tetrad showing the bilateral arrangement of microspores. 



The question as to which of the two modes of tetrad formation 

 is primitive and which is the more advanced is difficult to decide. 

 It seems, however, that since a division by furrowing is common 



9 The mode of origin of this gelatinous layer has been a subject of much discus- 

 sion. Beer (1906), Gates (1925), and Castetter (1925) have expressed the view 

 that it is secreted by the cytoplasm of the mother cell, while Farr (1922), Bowers 

 (1931), and Capoor (1937a) believe that it is the result of a swelling of the secondary 

 lamellae of the cell wall. 



