OF ARTS AND SCIENCES. 201 



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 there is a thickening which takes on a deeper stain than the com- 

 paratively thin walls do. In Figure 23, also, one of these thicken- 

 inofs shown in surface view is seen to be in the form of a rinji. In 

 cells which have separated more, and have a connecting hypha, there 

 appears to be just such a thickening at the point in the hypha which 

 represents the place of contact in the originally adjacent cells. 



On the longitudinal walls of these Inner cells of the inner cortex 

 may also be seen small rounded protuberances which grow out into 

 hyphoe. These hyph«, which in contrast to the connecting hyphae 

 may be called free liypha3, grow horizontally inwards, and twine 

 among the separated elements of the medulla. They sometimes arise 

 from pits, and probably always do, although this was not to be plainly 

 seen in all cases. The hyphae when just starting do not seem to have 

 a thickened ring at the base, but nearly all of the longer ones do. 

 There seems to be no fundamental distinction between free liyphiB and 

 connecting hyphte. A connecting hypha may be separated from one 

 of the two cells, either at the middle or at one end, and, thus becoming 

 a free hypha, continue its growth. Where, too, the free hyphae origi- 

 nate as small protuberances, it is probable that at these points had 

 been situated pits, but that the two cells had nevertheless entirely 

 separated from each other. 



The reason why the cells of the medulla and inner cortex become 

 separated from one another is not at all apparent. It is suggested 

 that these cortical cells, growing rapidly in length, become longer than 

 the corresponding region of more external cells, and so, becoming 

 somewhat bent, have a tendency to separate except at places where 

 they are most firmly attached, viz. at the pits. This would seem to 

 go a step farther in the medulla than it does in the inner cortex. In 

 the former the cells separate almost entirely from one another, except 

 as they are united together by the connecting hypha3, and, the ends 

 being free to slide up or down, the elements can elongate to a much 

 greater extent than they could when they were members of definite 

 vertical rows. 



We find, then, at the end of this period, four sorts of cells in a 

 medulla of this stage, viz.: first, the original tubes; second, the tubes 

 arising from the elongation of the cells separated off from the inner 

 cortex; third, the connecting hyphoa ; and fourth, the free hyphre. 

 The further changes in the medulla introduce no new kind of ele- 

 ments, but are brought about by the addition of new elements of the 

 last three kinds, and the changes which these undergo. 



The development of the cryptostomata may be readily investigated 



