DIATOMS. 75 
cell, followed by the pyrenoid, passes into the isthmus or connecting 
portion of the dumbbell-shaped zygote, which soon becomes cylindri- 
cal or crescent-shaped, and. scarcely a trace of the small nuclei are 
to be seen (Fig. 23, H). During the development of the zygote into 
an auxospore, the two large functional nuclei assume the structure 
characteristic of the resting stage (7. e., each presents a granular frame- 
work and a definite nucleolus) and fuse. The fusion does not take 
place in every case at a certain developmental stage of the two auxo- 
spores, but may occur earlier in one than in the other (Fig. 24, I, J). 
As a rule, however, the fusion is complete when the siliceous valves 
have begun to develop. The behavior of the small nuclei would seem 
to indicate that they are utilized as food. 
A slightly different process, leading to the production of the auxo- 
spore, is met with in Cocconezs placentula Ehr., as described by 
Karsten (1900). In this species the protoplasts of the conjugating 
cells do not divide, and, therefore, only one zygote results. In each 
cell there is also but one division of the nucleus instead of two as in 
Rhopalodia. Preparatory to the cytoplasmic union the protoplast of 
each cell contracts. Each cell is seen to possess two nuclei, one large 
and one small, so that nuclear division must have taken place at an 
earlier stage. During the contraction mentioned each protoplast sur- 
rounds itself with a gelatinous envelope. Near the point of contact of 
the two individuals the two halves of each shell separate slightly. From 
the opening in one of the cells, which is regarded as the male gamete, 
a small papilla protrudes, which grows toward the opening in the 
female cell, and the gelatinous envelopes are soon in open communi- 
cation. The entire protoplast of the male cell now passes through 
this narrow channel into the female cell. The young zygote then 
increases considerably in size, and begins the formation of a firm cell- 
wall about itself. Of the four nuclei only the two large ones are now 
to be seen, the smaller ones having gradually disappeared. The two 
_ large functional nuclei, each with a nucleolus, begin to fuse slowly, 
and, by the time the shell of the zygote is fully formed and the two 
chromatophores are reduced to one, fusion is complete. 
From the foregoing it is clear that the nuclear behavior immediately 
preceding the sexual act in Rhopalodza is strikingly analogous to 
the process following fecundation in Closterium and Cosmarium. 
Whether these processes bear any closer relation to each other than 
mere analogy is a difficult question. It may be suggested that, in the 
case of the diatoms, we have to do with the development of two perfect 
gametes in each cell instead of four, a process similar to that in certain 
Fucacee, where only part of the egg-cells in the odgonium mature, 
