108 



Bacillarieae 



takes place before conjugation (consult fig. 77 B D\ so that the conjugating 

 cells have two nuclei, one of which is large and one small. During con- 

 jugation the large nuclei fuse whereas the smaller ones gradually disappear. 

 Conjugating cells 18 19 yu, in length and 12 13 fi in breadth produced 

 auxospores 40 41 /j, in length by 28 33 /j, in breadth. In Surirella saxonica, 

 Karsten found that division of the nucleus again occurred previous to conju- 

 gation, but that four nuclei were formed in each cell. These were at first all 

 of the same size, but soon one of them grew much in size, forming a large 

 nucleus as compared with the other three. On conjugation the large nuclei 

 of the gametes fuse, and the six small nuclei presumably disorganize, as only 

 the fusion-nucleus is found in the fully-formed auxospore (fig. 77 E). 



(4) In some cases two diatom-cells become enveloped in a common 

 mucus and the protoplasts throw off the old cell-walls, but there is no 

 conjugation. The two protoplasts generally lie close together and each 

 develops independently into an auxospore. This type, in which each gamete 

 undergoes rejuvenescence without conjugation taking place, is not in- 

 frequently observed in Cocconema. 



(5) As in the previous type a pair of cells become enveloped in a 



common mucus, and the protoplast of each 

 cell forms two gametes by division. The 

 gametes of the two cells fuse in pairs forming- 

 two zygotes, each of which develops into an 

 auxospore. This type has been observed in 

 Amphora ovalis,Epithemia Argas,RJiopalodia 

 gibba, Navicula limosa (fig. 78.4), and a few 

 others. The cytology of the conjugation has 

 been followed out by Klebahn ('96) in the 

 case of Rhopalodia gibba. The protoplasts 

 of the two cells become closely approximated, 

 and the nucleus of each protoplast divides 

 first into two and then into four. Two of 

 these nuclei soon become reduced in size 

 while the other two are conspicuous by their 

 larger size. Each protoplast now divides 

 into two portions, each of which contains 

 two nuclei, one large one and one small one. 

 The halves of the approximated protoplasts 

 now coalesce, the larger nuclei fusing while 

 the smaller nuclei gradually disorganize 

 (fig. 79). The important feature thus brought 

 to light by Klebahn is the division of the 

 nucleus of the original protoplasts into four, 



Fig. 78. A, Navicula limosa Kiitz. 

 ( x 450). B, Achnanthes flexella 

 (Kiitz.) Br6b. ( x 450). C, Navicula 

 Amphisbsena Bory ( x 450). D, N. 

 viridis Kiitz. (x 350). C and D 

 illustrate the first method of auxo- 

 spore- formation , B the third method, 

 and A the fifth method. 



