Age Determination, Growth and Symmetry 



29 



That the yellow layer increases only by additions at the margin of 

 each plate is indicated by the following facts. The cells are arranged 

 in a fairly regular manner. They are as numerous in the newer portions 

 of the plate as in the older. When a zone, presumably annual, of a plate 

 has been completed, the yellow layer ceases to increase appreciably in 

 thickness. THe older portions of the yellow layer show no intercalations 

 of incompletely degenerated cells with distinct zone of newly formed 

 yellow substance, although such cells occasionally appear on the inner 

 side of the yellow layer. The zone of the plate in process of formation, 

 that is the one next the margin, is not so yellowish in colour as the older 

 zones. In section the yellow layer in this zone is thinner toward the 

 turned-in margin of the plate, and is absent from the bottom of the 

 groove separating this plate from the neighbouring one, although, as is 

 apparent in a surface view, the cells are not less numerous toward this 

 margin. It would appear that at the growing margin certain cells reach 

 the surface and begin the production of the yellow substance, which 

 production is completed by the cells before a new season of growth has 

 begun. When the available space has been occupied by cells, no others 

 can reach the surface and further production of yellow substance is 

 prevented. The production of yellow substance by the marginal cells 

 apparently does not cease when the margin fails to extend, so that the 

 full thickness of the yellow layer is attained at the margin before the 

 growth of another season begins, and thus the turned-in condition of the 

 margin at the winter stage becomes set. 



By means of the lines of growth we have calculated the ages of the 

 Alaskan specimens and we give them herewith, the numbers in each case 

 including the uncompleted year. 



There is considerable variation in size among those of the same age. 



In those over one year in age we have calculated the sizes of the 

 various winter periods in their history in the following manner. Several 

 plates were selected in which the lines were particularly distinct. The 



