NUCLEAR CYCLE IN CHLOROPHYCEAE VE 
in the Diatom immediately precedes conjugation, while in the Desmid 
it immediately fo//ows on conjugation. This fact is important for comparison 
with the order of events in other Thallophytes. 
The post-sexual phase in Ulothrix, Oedogonium, and Coleochaete has 
been frequently brought into comparison with that of the Bryophytes, 
but the necessary details are still very imperfect as regards their nuclei. 
In Ulothrix the zygote is formed by the fusion of two motile gametes, 
and after encystment it undergoes a period of rest: parthenospores similar 
to.the zygotes are also produced, as shown by Klebs,! when the sexual 
cells are exposed to a 0'5% culture solution: these also undergo a period 
of rest.. On germination it is found that in either case there is a division 
of the contents into non-motile cells, which grow directly into new filaments. 
But apparently there is this difference, that the parthenospores form only 
two of these, while the zygotes form four. If this be constantly so, the 
point has a special interest as indicating that the tetrad-division is a 
consequence of sexual fusion, and need not happen unless the fusion be 
carried out. In a similar way the zygote of Oedogonium divides on germina- 
tion into four, a number which suggests reduction; it is quite possible 
that the exceptions to this noted by Pringsheim,? may be explained on 
the ground of parthenogenesis, as in Ulothrix. The cell-body formed on 
germination of the zygote of Coleochaete is that which has most frequently 
been compared with the simplest sporogonia of Bryophytes. In this plant 
nuclear fusion has been observed in the ovum (Fig. 42. 6, 7), after which 
the zygote undergoes a period of rest, as a unicellular, uninucleate body, 
covered by a pseudoparenchymatous coat. On germination a transverse 
wall is formed at right angles to the axis of the oogonium: then follow 
longitudinal walls to form octants, and these usually divide further till 8-16 
cells are formed in each hemisphere. The outer coat then bursts about 
the region of the transverse wall, and a zoospore is formed from each 
of the cells, which germinates liké any ordinary zoospore. The question 
of homology of this body with a simple sporophyte has recently been 
decided by observations made by C. E. Allen®: he found that reduction. 
takes place, with characteristic synapsis, on the first nuclear division in 
the germinating zygote. The cells of the “fruit-body” are then haploid, 
and correspond cytologically with the gametophyte, not with the 
sporophyte as had been commonly assumed. But there is no need 
as an alternative to hold this body as in any sense a derivative of the 
ordinary Coleochaete thallus: it seems more natural to see in it merely 
an extension of the usual tetrad; for if the second division of a normal 
tetrad were in this case repeated twice or thrice, a cell-group would 
result just as is seen in Coleochaeze, and the biological advantage of increase 
in number of the progeny would thus be secured. Morphologically such 
1 Bedingungen der Fortpflanzung, p. 321. 
2 Gesammelte Abhandlungen, i., p. 251. 
3 Ber. a. D. Bot. Ges., 1905, p. 285. 
