MICIIICAX ACADEMY Ol" SCIKNCR. 71 



those in a honeycomb. This idea may be traced to Hertwig and Lesser 

 ('71), wlio, after considering and rejecting former views, declare posi- 

 tively: "Ihrer feineren Struktur nacli besteht die Schale aus zwei Plat- 

 ten, einer j'lusseren und einer inneren, welclie einander parallel gclagert 

 sind, und durch cin bienenwabenartiges hexagonale Figurcn bildendes 

 Fachwerk vereint werden." 



The same authors treated the shell with sodium carbonate and acetic 

 acid, whereby bubbles were formed in the cancelli, showing them to be 

 hollow. The fact that the cliambers Avere closed above and below has 

 never been questioned until recently. Cushman and Benedict (06) are 

 of the opinion that there is oiily one membrane, the inferior one, so that 

 the cancelli are open above. They say that by exposing the shell to the 

 air, bubbles collect in the chambers without further treatment. Excellent 

 photomicrographs showing this result are included in the work, and they 

 state that in cross sections no superior membrane appears. On the con- 

 trary, however, the later careful work of Khainsky ('10) indicates both 

 membranes in sectioned |)reparations. It is barely possible that both 

 are right. If the original cauctlli wire constructed witli delicate end 

 plates, it is conceivable that these might later be ruptured or dissolved. 



Aside from this point, the fine anatomy of the shell seems to be com- 

 plicated beyond the conception of the earlier writers. Cushmann and 

 Benedict (06) give pliotomicrographs which sliow clearly that the struc- 

 ture is not the simple hexagonal one usually figured. In the typical 

 liexagonal structure, all six sides are in contact with similar sides of 

 other liexagons, while in the shell pattern of Arcella, the sides of the 

 liexag(ms are never in contact with each other, but with smaller triangular 

 figures. The scheme may be reproduced by drawing tln'ee sets of parallel 

 lines at an angle of sixty degrees with each other. 



Orkfin and Formation. The manner of construction of this compli- 

 cated shell is worthy of note. For a long time, it has been a common 

 observation 'that new Arcellas were formed by fission and constructed 

 a new shell from intrinsic material. This rudiment of the shell is so 

 flexible and transparent that it has generally been held to be structureless. 

 Khainsky, however, discusses certain intra vitam stains which render the 

 structure visible during formation. He states that after the emergence 

 of the protoplasm from the cavity of the maternal shell, a clear delicate 

 membrane is secreted around the projecting plasma. This under sufficient 

 magnification exhibits an irregular foam structure, with larger and 

 smaller alveoli promiscuously intermingled. At this time, these are plas- 

 tic and flexible. As growth jiroceeds, due to side pressure they become 

 higher and assume the typical hexagonal shape, with the smaller alveoli 

 filling the interstices between the larger. At this time the sh^ll of the 



