gS ANDREWS. 



The length of striations varies to any extent, and may 

 do so independently of a quite stable linear arrangement of 

 vesicles along their course. It will vary in a series or in a 

 single stria from moment to moment. Striae may lengthen 

 or shorten rhythmically or intermittently. During shortening 

 they may become more tenuous, delicate, and finally less viscid 

 and refractive, and even disappear from sight or from all but 

 the closest scrutiny. Or they may become thinner and more 

 obvious optically, because more refractive. 



They may become thicker and more refractive. Those 

 striae which I have distinguished above as true fibrils, become 

 always thicker and more refractive during shortening contrac- 

 tion, and thinner and more refractive in extension. But striae 

 may lengthen actually by implication of new substance and 

 not by mere displacement of already organized substance. 

 They may shorten likewise by relaxation of such organization 

 along lines once emphasized. In other words, the physical 

 conditions in the finer foam on which they depend may 

 be as unstable as similar organizations in the structure of 

 Biitschli. Such phenomena as these latter are best seen in 

 the development of eggs, where the elements are constantly 

 undergoing metamorphoses of structural arrangement. 



Striae may travel from place to place without visible dis- 

 turbance of an existing structure of Biitschli, or arrange- 

 ment of its elements. In such case, they progress either as 

 a wave of influence upon interalveolar substance, or as actual 

 displacement of this along interalveolar lines. 



It is common in contractile tissues, or areas, whether stable 

 or unstable, among Protozoa, and also Metazoa, such as hydra, 

 rotifer, frog, and starfish and sea-urchin embryos and larvae, 

 to find ladder-like arrangements of the vesicles, of various 

 lengths, extending in a number of directions with respect, 

 not to a common centre, but to many centres, and as it were 

 growing out of each other at various angles ; the optical or 

 linear emphasis beginning or ending at any point, with or 

 without extension of the alveolar walls of the structure on 

 which it is marked out. A kindred incoherence of divergent 

 emphases may be seen in the unsegmented o^gg of starfish and 



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