STRUCTURE.] A SEXUAL ELEMENT POSSIBLE. 131 



in the lowest of all Algals, Brittle worts, or Diatomaceae : 

 " In many of the Diatomaceae," he says, " it is seen that at 

 a certain period of the development of the species a union of 

 the endochromes of two distinct frustules seems necessary for 

 the continued existence of the species as well as for its repro- 

 duction. The physiologist will endeavour to arrive at some 

 probable explanation of the reason why this mixture of 

 endochromes is necessary, and he will feel it difficult to come 

 to any other conclusion than this : namely, that in each of 

 the conjugating endochromes an essential element must to 

 some extent, probably very trifling, be wanting, whilst another 

 essential element is in excess ; and that a mixture of such an 

 endochrome with another similarly conditioned, except that 

 the quantities of such respective elements are reversed, must 

 take place, in order to restore the equilibrium and enable the 

 species to continue its existence. The circumstance of the 

 mixed endochrome developing around itself a cell-wall pre- 

 cisely similar in every respect, except in size, to that of the 

 ordinary frustule, would seem to indicate very slight, if any, 

 difference in the qualities of their respective endochromes. 

 The sporangium, the product of this mixed endochrome, 

 undergoes fissiparous division, too, in like manner with the 

 ordinary frustules, and is thus converted into a number of 

 sporangial frustules. In what way the small ordinary frus- 

 tules are produced from these has not yet been observed." 

 (Annals of Nat. Hist., i. 162, n. s.) 



