1920] SHARP— SPERM A TOGENESIS 259 



centrosomes shown in fig. 1, after the manner of the " black gran- 

 ules" in the body cell of Dioon (Chamberlain 4). On the other 



hand, it would be possible to select a series of cells illustrating the 

 divergence of daughter centrosomes arising by the division of one, 

 as in Equisetum (Sharp 8) ; or even to show the origin of the bodies 

 in question from the nucleus, as described by Wilson (10) for 

 Atrichum and Mnium. The writer, however, believes that the 

 evidence afforded by his material is insufficient to support any 

 of these hypotheses in the case of Blasia. The present descrip- 

 tion, therefore, will begin with a stage (fig. 1) at which the identity 

 of the centrosomes is unmistakable, the question of their origin 

 and earlier history being left an open one. 



Two centrosomes, whatever may be their previous relation to 

 other cell granules, soon stand out with great distinctness as 

 intensely staining bodies near the cell membrane at opposite poles 

 of the androcyte mother cell (fig. 1). At this time the cell is still 

 rather square in section, since it has only begun to round off from 

 its neighbors, and the centrosomes commonly occupy the corners, 

 as show r n in the figure. From each centrosome a conical group of 

 very faint fibers extends toward the nucleus, which is somewhat 

 flattened on the sides facing the centrosomes. While the nucleus 

 is undergoing the prophasic changes (fig. 2) these fibers become 

 more plainly visible, and when the nuclear membrane disappears 

 they become attached to the chromosomes and establish the 

 achromatic figure with the centrosomes at its poles. 



It is at metaphase that the spindle is seen most clearly (fig. 3). 

 As noted by Woodburn (12), it may lie either straight or obliquely 

 in the cell. Furthermore, the cells may round up and alter con- 

 siderably in shape while mitosis is in progress, so that although 

 the centrosomes may at first be situated near the corners of the 

 cell, all appearance of the diagonal division so characteristic of 

 many bryophytes may in many cases be lost by the time the meta- 

 phase and succeeding stages are reached (figs. 4, 5). 



When the chromosomes reach the poles at the end of the ana- 

 phase (fig. 4), they usually come in contact with the centrosomes. 

 As a result the latter, which are very minute, are often difficult to 

 find at this stage. Careful search, however, reveals cells in which 



