1902] A MORPHOLOGICAL STUDY OF THUJA 253 



About the time of the cutting off of the neck cells, the 

 jacket cells appear. At first they are poor in contents, but soon 

 become filled with dense cytoplasm, and the adjacent layer of 

 prothallial cells also shows a marked increase in activity. Proto- 

 plasmic connections between the jacket cells and the central 

 cells were not seen, but it is possible that they were overlooked 

 owing to the extreme thinness of the dividing wall. It is not 

 believed that the nuclei of the jacket cells pass into the central 

 cells, as reported in Cephalotaxus by Arnold! (i). 



Shortly after fertilization the jacket cells break down, and in 

 many cells the chromosomes become separated, and are then in 

 a favorable condition for counting, the gametophyte number 

 being twelve. 



After the neck cell has been cut off, the nucleus of the central 

 cell lies very close to the upper end of the cell [fig, 10^ and 

 enlarges so much that its longer diameter is sometimes more 

 than four-fifths of the diameter of the ^gg- A large vacuole 

 {figs. lo-ii) now rounds out in the center of the cell. Cyto- 

 plasmic masses collect at each end of the central cell, and are 

 connected with each other by a thin layer at the periphery of 

 the central cell. Two deeply staining masses, presumably 

 kinoplasmic in nature, are present. The one near the nucleus 

 resembles somewhat the mass mentioned in connection with the 

 body cell, but it is not so sharply limited. The other, in the 

 lower end of the egg, sometimes stains so densely that it often 

 resembles an overstained nucleus. 



At this stage the tip of the pollen tube is in the mouth of the 

 archegonium complex, and is separated from the central cells by 

 remnants of the neck cells. 



THE VENTRAL NUCLEUS. 



Karyokinetic figures showing the cutting off of the ventral 

 :leus were obtained in material fixed June 20, 1901, and June 



(fig. 12). It takes a position at one side of and a 



little above the tgg nucleus, almost in contact with the wall of 

 the G^g, and usually on the side nearest the center of the arche- 



