DIATOMEJ:. 



19 





\ 



into the smaller valve of the original frustule. The latter cell 

 (frustule) is thus, upon the whole, ^ -g 



smaller than the mother-cell, and as 

 the cells do not increase in size, some 

 frustules are smaller than the ones 

 from which they are derived, and thus, 

 by repeated divisions, it follows that 

 smaller and smaller frustules are pro- 

 duced. This continued diminution in 

 size is, however, compensated for by 

 the formation, when the cells have 

 been reduced to a certain minimum, 

 of auxospores, 2-3 times larger. These 

 may either be formed asexually by the 

 protoplasm of a cell increasing, round- 

 ing off and surrounding itself with a 

 new wall (e.y. Melosira} or after con- 

 jugation, which may take place with 

 various modifications: 1. Two indi- 

 viduals unite after the secretion of a 

 quantity of mucilage, and the valves 

 then commence to separate from each 

 other, on the side which the two indi- FlG - ^--P^nuiaria .- B, from 



. , . the edge, shows the valves fitting 



viduals turn towards each other. The together; .4, a valve. 



FIG. 15. Various Diatomacese. A Diatoma vulgare. B Tubellaria flocculosa. Navicula 

 tumida (lateral views). D Gomphonema const ric twin, (lateral views). E Navicula u-estll 

 (lateral views). 



protoplasmic bodies now release themselves from their cell-wall, 

 and each rounds off to form an ellipsoidal mass ; these two pro- 



