452 PHYSICAL GEOGRAPHY. 



by which they have been generated, instead of across them, as is the case 

 in EpitJicmia. Although in many cases the frustules, which are about to 

 conjugate and form sporangia, split into two separate parts, so that their 

 contents may freely coalesce, we find that there are examples where the 

 valves only split apart at one end to a slight extent, but enough to serve 

 for the escape of the endochrome. Instead, also, of the pair of conju 

 gated frustules, producing between them two sporangia, they may develop 

 but a single one. The Melosirce and BiddnlpJiue (the former looking like 

 a string of pill-boxes attached together by their tops and bottoms ; and 

 the latter beins: somewhat like a number of wool-sacks united at the cor- 



o 



ners into a chain), Mr. Thwaites remarks, &quot;would seem in their develop 

 ment of sporangia to offer an exception to most diatomacea; ; for, in those 

 genera, no evident conjugation has been seen. However, something anal 

 ogous to it must take place; for, excepting the mixture of endochromes 

 of two cells, the phenomena are of precisely similar character. Thus, 

 instead of the conjugation of two frustules, a change takes place in the 

 endochrome of a single frustule; that is, a disturbance of its previous 

 arrangement, a moving towards the centre of the frustule, and a rapid 

 increase in its quantity ; subsequently to this it becomes a sporangium. 

 In a single cell, therefore, a process, physiologically precisely similar to 

 that occurring between two conjugating cells, takes place; and it is not 

 difficult to believe, taking into view the secondary character of cell- 

 membrane, that the two kinds of endochrome may be developed at the 

 opposite ends of one frustule as easily as in two contiguous frustules, 

 and give rise to the same phenomena as ordinary conjugation.&quot; In such 

 genera as have their siliceous frustules enclosed within membranous 

 tubes, as ScJiizoucina, conjugation seems to take place both without and 

 within the tube, but oftenest upon the outside. As has been remarked, 

 &quot;one reason for the paucity of observations on this process in the dia- 

 tomaceae is no doubt to be found in the changes which usually take place 

 in the condition of these organisms at this period of their existence. 

 During conjugation the process of self-division is arrested, the general 

 mucus envelope or stratum, produced during self-division, is dissolved, 

 and the conjugating pair of frustules become detached from the original 

 mass; they are thus more readily borne away and dispersed by the sur 

 rounding currents, or the movements of worms and insects, and their 



