1 76 Campbell— On the Development of 
cell is formed, but the first division separates the neck from 
the central cell (Fig. 56). The neck-canal-cell is formed much 
earlier than given by Pringsheim for Salvinia 1 , and is cut off 
from the central cell about the time that the vertical walls in 
the neck-cell are formed. The wall by which the neck-canal- 
cell is separated from the central cell, is concave, but becomes 
nearly straight as the neck lengthens. . Each of the four 
primary neck-cells divides into four as in Salvinia , and like 
it, the divisions are completed before any marked elongation 
of the neck is noticeable. The ventral canal-cell is cut off 
by a very strongly curved wall, and sometimes, if not always, 
before the divisions in the neck-cells are completed (Fig. 58 b). 
The nucleus of the neck-canal-cell may undergo division, 
very much as in the homosporous Filicineae and Isoetes 
(Fig. 58). This has not been recorded for Salvinia. Whether 
it always occurs in Azolla it is impossible now to say. After 
the divisions are completed the neck rapidly lengthens and 
projects strongly above the surface of the prothallium, resem- 
bling much more that of the homosporous Filicineae than 
it does that of the Marsiliaceae. 
A curious fact noted was the retention for a long time of 
an apparently normal structure of the protoplasm and nucleus 
of the central cell of the unfertilized archegonia. Instead of 
shriveling up, and the walls of the central cell turning brown 
as is usually the case, the cell remained turgid, and the 
protoplasm and nucleus retained much the same structure 
as in the freshly opened archegonium. Indeed it was often 
impossible to tell whether an archegonium had been fertilized 
or not. 
The prothallium seems to have very little power of in- 
dependent existence, and develops but little chlorophyll even 
in the older unfertilized ones. No root-hairs were observed in 
any case, and the limit of its' growth is probably determined 
by the amount of food material in the spore. The number 
of archegonia may occasionally exceed twelve, and from 
1 Goebel : Outlines, p. 232. 
