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 (/. 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 Cocconeis 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 Rhopalodia 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 

 Fucacece, where only part of the egg-cells in the oogonium mature, 



