248 Evans: Air chambers of Grimaldia fragrans 



in size which these chambers show, when compared with the small 

 intercellular spaces in Fig. id, is due to the vigorous growth of 

 the bounding cells, accompanied by rapid cell divisions. At this 

 stage the partitions show no evidence of outgrowths. Fig. 12 

 represents the section just below the one shown in Fig. ii, the 

 cells drawn, in part at least, forming the floors of the dorsal cham- 

 bers. The figure shows many intercellular spaces, which are 

 clearly schizogenous in origin; these spaces represent the begin- 

 nings of the more deeply situated chambers or, in some cases, the 

 passageways leading from these chambers to the dorsal chambers. 



A longitudinal section, repre- 

 senting about the same stages as 

 those shown in Figs, ii and 12, 

 may be seen in Fig. 13, the left- 

 hand side of the figure being 

 toward the apical cell. The very 

 rapid development of the cham- 

 bers is clearly indicated, and light 

 is thrown on the way in which 

 the deeper chambers originate, 

 such chambers being indicated by 

 Fig. 13. Longitudinal section through ^^^ letters a-g. It wiU be seen 

 young chambers, X 500. a-e, more , , , • , , 



deeply situated chambers. ^hat some of these chambers 



seem to be completely enclosed, 

 showing that they may have originated by a splitting of cell walls 

 in compact tissue, and that others already communicate with 

 more dorsally situated chambers. Whether the connecting 

 passageways are always formed subsequently to the chambers, 

 or whether the formation of the passageways may sometimes 

 precede that of the chambers is not altogether clear. If the older, 

 right-hand side of the figure is compared with the younger, left- 

 hand side, it becomes evident that the tissue with intercellular 

 spaces has almost tripled in thickness and that the dorsal chambers 

 have become distinctly deeper. The rapid growth involved in 

 these changes has taken place in the original partitions of the 

 dorsal chambers, in the cells which formed their irregular floors 

 and in the cells immediately beneath. As the writer conceives 

 the process, the growth of the partitions is both horizontal and 



