340 Davis —On the Development of the Frond of 
the structure and manner of growth of this alga and allied 
species had been published by Nageli, Berthold, and Wille. 
It will be of interest to compare later their views with those of 
Debray and Bigelow. Debray and Bigelow arrived inde- 
pendently at the same conclusions in regard to the apical 
growth and the development from the growing-point of the 
structures characteristic of the adult frond of this species. My 
own observations are in accordance with their conclusions, 
and a brief resume of their investigations had best be given 
here. 
They found that the tip of the adult frond was occupied by 
a group of actively growing cells. These cells were often dis- 
posed in three distinct circles. The inner circle was composed 
of four to six cells, which met at a point at the apex. Each 
of these cells gives rise to a file of segments, which is cut off 
behind it, and which can be traced for some distance from its 
proper initial cell. These are the primary files of segments. 
Just behind the initial cells of the first circle is a second circle 
of initial cells, which usually have their pointed ends wedged 
in between the initial cells of the first circle. Each initial cell 
of the second circle gives rise to a file of segments. These 
are the secondary files. Again, just behind the second circle 
and between the primary and secondary files of segments, is 
a third circle of initial cells, double the number of the first or 
second circle. Each of these cells also gives rise to a file of 
segments. 
The segments cut off from the initial cells soon undergo 
a very important division. This usually occurs when the 
segment is separated from its generating cell by one sister- 
segment. A cell-wall is formed across the segment perpen- 
dicular to its greatest diameter and parallel to the surface of 
the frond. The segment is thus divided into an inner and an 
outer cell. 
The cortex of the frond is developed from the outer cells of 
the segments. These divide irregularly by cell-walls perpen- 
dicular to the surface of the frond. This division first begins 
when the segment is separated from its initial cell by from 
