1 72 Campbell. — On the Development of 
quite as large and closely resembled it. In this respect Azolla 
offers a strong contrast to Marsilia 1 , where the prothallial 
nucleus is much larger than that of the spore. How it com- 
pares with Salvinia in this respect can only be known after 
stained sections of the latter have been studied. Berggren 2 
failed to demonstrate the presence of a primary division-wall, 
and figures the earlier stages of the prothallium as having the 
lower cells of very indefinite form with no distinct wall sepa- 
rating them from the spore-cavity. The first division-wall in 
the prothallium seems to correspond with that in Salvinia. 
This is a vertical wall (Fig. 44 /-/), and divides the cell into two 
cells of unequal size. In Salvinia , according to Prantl 3 , the 
former cell remains sterile, while in Azolla it also may produce 
archegonia, although later than the rest of the prothallium. In 
a very young prothallium, having but three cells (Fig. 43), the 
next wall was also nearly vertical, but in other cases (Fig. 46) 
it looked as if this were not always the case, but that, as in 
Salvinia 4 , the second wall was horizontal and divided the 
larger cell into two nearly equal ones. From the upper one 
the first archegonium is developed at a very early stage. Its 
position varies a good deal, depending apparently upon the 
position of the first division-wall in the prothallium, and also 
upon the time when the first horizontal wall is formed. If the 
latter is formed early, the first archegonium is nearly central, 
but if this is not formed until after two vertical walls have 
been produced, the archegonium is nearer the side opposite 
the first cell cut off. In the few cases where successful cross- 
sections of very young prothallia were made, the archegonium 
mother-cell was decidedly triangular in outline, indicating that 
it is cut off by the walls meeting at nearly equal angles 
(Fig. 52). It is easily distinguished in the very young pro- 
thallium by its denser contents that stain more strongly than 
those of the surrounding cells. The archegonium-mother- 
cell divides now into two by a transverse wall, the lower of the 
1 Campbell. On the Prothallium and Embryo of Marsilia vestita, Proc. Cal. 
Acad. Sci. 1892, p. 196. 
2 loc. cit. p. 2. 3 Prantl, loc. cit. p. 427. 4 loc. cit. p. 429. 
