Febeuaey 11, 



i.] 



SCmNGE. 



201 



becomes so rapid that photographs exposed 

 one-fiftieth of a second show blurring, due 

 to movement. On changing the red light 

 to violet or mild white, streaming instantly 

 stops and sometimes reverses. Swinging in 

 green, red or yellow screens causes the flow 

 to be resumed after an interval varying in 

 dififerent individuals from an almost imper- 

 ceptible minimum to ten seconds. The fol- 

 lowing effect of any color was generally 

 constant after the same preceding color, and 

 as stimulants to flow the colors increased 

 in effectiveness as one approached the red 

 end of the spectrum, while as retarders of 

 flow white light and the colors at the act- 

 inic end were most powerful. 



The preceding experiments were per- 

 formed by means of a large photomicro- 

 graphic apparatus, the image of the Am,(eba 

 being projected by an arc light upon the 

 ground glass back. What little heat there 

 was [24.8° C] was equalized for the diifer- 

 ent colors by mica screens. Intensity was 

 eliminated by adding more color screens, 

 ■which diminished the brightness but seemed 

 to accentuate the characteristic color efiect, 

 whether it was a retarding or a stimulating 

 efiect. 



to the anal segment, and the future elonga- 

 tion of the stolon takes place by the sepa- 

 ration of new segments from the anal seg- 

 ment. 



The separation of the stolon takes place 

 in a region of embryonic tissue which does 

 not form part of a true segment, but which 

 is derived from the undifferentiated tissues 

 of the anal segment. 



A wide range in the position of the chain 

 exists in this species. In young specimens 

 the chain is as far forward as the 19th seg- 

 ment of the parent stock, while in older 

 and larger specimens it is placed as far pos- 

 terior as the 59th segment, certain charac- 

 ters in the embryonic region of the chain 

 indicating that, besides being active in the 

 formation of stolons, this region also adds 

 segments to the parent stock. 



The cycle of stolonization in this species 

 is: (1) The development of a first stolon 

 on the young asexual individual by a process 

 akin to fission. (2) The development of a 

 chain of stolons from the last segment of 

 the parent stock by budding. (3) The de- 

 velopment of possibly a single stolon pos- 

 terior to the middle region of the parent 

 stock by true fission. 



Stolonization in Autolytus varians. P. C. 



Mensch. 



As many as eight individual stolons have 

 been observed in single chains of this 

 species. The embryonic segments forming 

 the stolons are derived as outgrowths from 

 the last segment of the parent stock, which 

 itself shows internal structures different 

 from those of preceding segments. This 

 process of segment formation contributes 

 three or four segments to the future stolon, 

 the posterior one of the series retaining its 

 embryonic characters and forming the anal 

 segment of the stolon. At the time the an- 

 terior of the three or four embryonic seg- 

 ments begin to thicken for the formation of 

 the head a new segment appears anterior 



The Use of the Centrifuge for Collecting Plank- 

 ton. G. W. Field. 



Hensen's counting method is the present 

 basis of quantitative and qualitative Plank- 

 ton determinations. Yet improvements are 

 desirable and feasible. The desideratum is 

 a practical, rapid, simple method capable 

 of general application, by which data can 

 be obtained for use in determining the com- 

 parative economic value of all waters, either 

 for scientific agriculture or for municipal 

 water supplies. Counting of individuals and 

 enumeration of species seem to be neces- 

 sary, together with an accurate estimation 

 of the volume of the inorganic matter and 

 of the organic amorphous debris. 



The chemical determination of the amount 



