234 Campbell . — On the development 
zoology. The old method of rendering the embryo trans- 
parent by caustic potash and similar violent agents, while it 
may enable one to get a general idea of the structure of an 
embryo, can never show with exactness the cell-arrangement 
in a many-celled embryo, owing to the inevitable confusion 
arising from trying to get optical sections where several super- 
posed layers of cells are present. At the same time the 
structure of the cell-contents is absolutely destroyed by these 
means. With freehand sectioning it is impossible to get more 
than a very few sections, indeed seldom more than a single 
good one of a young embryo, and of course only a partial idea 
of its structure is thus obtainable. 
Hofmeister’s brief account of Pilularia 1 , while in some par- 
ticulars correct, is on the whole very imperfect, and the same 
may be said of Hanstein’s work 2 . The later work of Arc- 
angeli 3 is much better, but is also in several particulars, 
notably the development of the male prothallium and the 
earliest stages of the female prothallium also, far from com- 
plete, and his account of the development of the embryo, 
as well as the figures of the same, leave very much to be 
desired. 
The material used in making the investigations here re- 
corded was obtained from the botanical garden in Berlin, 
where, in the autumn of 1887, Pilidaria was growing luxuri- 
antly and had formed great numbers of ripe fruits. These 
were gathered at different times up to the middle of December, 
and placed in ordinary unglazed earthen pans filled with 
earth. They were kept in the cold-house connected with 
the laboratory, and retained their vitality perfectly as long as 
the observations lasted, all that was necessary being to water 
them moderately from time to time. In this way an abundant 
supply of fresh material was kept on hand all winter. It was 
found that if the spores were allowed to become perfectly dry 
for any length of time, that many of them, especially the 
1 Vergleichende Untersuchungen. 
2 Pilulariae globuliferae generatio cum Marsilia comparata. Bonn, 1866. 
3 ‘ Sulla Pilularia e Salvinia in Nuovo Giornale Botanico Italiano, viii. p. 320. 
