l$6 BOTANICAL GAZETTE [march 



OOGENESIS 



The nuclei in the cells of the cushion region are generally larger 

 than those of the rest of the prothallium. At first, successive mitoses 

 that occur in the region to become the cushion divide cells anticlinally, 

 so that there result a number of narrow parallel cells in a single row. 

 In one of these cells, the next mitosis is periclinal and gives rise to an 

 archegonium initial (fig. 34). From the archegonium initial, by two 

 successive mitoses, there are cut off a basal cell, a primary neck cell, 

 and a central cell, or primary oogenous cell (figs. 35, 36, 37). Up 

 to this differentiation of the central cell, the critical stage of each 

 mitosis showed that the number of chromosomes is 64 or 66. 



The central cell grows in size even after its nucleus begins to divide 

 and becomes characteristically asymmetrical in form, due to the 

 influence of the surrounding cells. The cytoplasm in the central cell 

 consists of a fine fibrillar structure in general, which passes into an 

 alveolar structure toward the periphery (fig. 38). There are observed 

 in the central cell thread structures of various lengths, scattered 

 throughout the whole cell, especially near the cell wall. Their stain- 

 ing capacity and general aspect recall similar structures which are 

 observed in the central cell of the antheridium. 



The resting nucleus in the central cell increases in size considerably 

 (fig. 38). The nuclear reticulum consists of ragged chromatin material 

 and three or more nucleoli are present. There now takes place a 

 mitosis to divide the cell into neck-canal and ventral cells (figs. 3Q-42). 

 The mitosis is typically vegetative; in the prophase the spirem is 

 continuous and a single nucleolus is present, perhaps resulting from 

 the union of the nucleoli in the resting stage. The appearance of 

 longitudinal fissions in the chromosomes, their shortening and their 

 arrangement in the equatorial plate, were exactly similar to those 

 which were described in the vegetative mitoses of the prothallia. In 

 the anaphase the two sets of daughter chromosomes show no difference 

 in the amount of chromatin material (figs. 41 } 41a), but after the 

 telophase the two daughter nuclei become unequal in size (fig. 42). 

 The inequality of these two sister nuclei has resulted neither from 

 unequal distribution of the chromatin mass nor from the number 

 of chromosomes, but is a matter of nucleo-cytoplasmic relation, i. e., 

 on account of the position of the nucleus of the central cell, the cell 



