14 



THE STRUCTURE AND LIFE 



valve over each divided surface ; in the place of the primitive original frustule 

 we therefore now find two frustules, each composed of a new and an old valve. 



Fig. 8 



r Fig. 8' Section of a diatom at the commencement of deduplication. A. Nucleus commen- 

 cing to divide, showing distinct nucleoles. B. Protoplasm. B.' Primordial utricle. C. 

 Endochrome. D. Central cavities. FF.' Valves. GG.' Connecting parts. 



pig. g Section of a diatom in process of deduplication. AA.' New nucleus and nucleoles. 

 BB.' Protoplasm. BB.', Double primordial utricles. CC. Divided endochrome. DD.' Central 

 cavities. F.' External mother valve. F. Internal mother valve. ft".' New daughter valves. 

 GG.' Connecting parts. (J. Deby). 



" While the new valves are being formed," writes Mr. Deby, " They 

 thicken and are covered with the markings peculiar to the species, and rapidly 

 take the form and appearance of the external valves ; this secretion appears 

 to take place from the centre towards the periphery (fig. 9). These new 

 valves inside the original frustule occupy a more or less central position in it 

 and face one another. 



" We now see the diatom made up of four valves, of which two are external 

 and old, and two internal, close together, new and attached to the entire inside 

 circumference of the old connecting parts ; at this period the young valves 

 are as yet without any connecting part. 



" Soon afterwards, sometimes in fact before the division of the primordial 

 utricle, it may be noticed that the connecting parts become considerably 

 enlarged and that at the same time the interior has slid into the exterior so as 

 to cause a greater space between the two external valves and to increase the 

 internal cavity of the frustule. The connecting parts of the young valves are 

 only developed later, either before or after their liberation, according to the 

 genera and species of the diatoms. A little later, in species whose frustules 

 are solitary, the sliding of the connecting parts attains its maximum, and the 

 narrower frustule liberates itself entirely from the other which before was 

 acting as a sheath to it. 



" From what has just been stated it follows that in the same species of 

 diatom, according to its stage of development, individuals may be met with 

 possessing 



