ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 303 



Rotatoria. 



Bdelloid Rotifers of South Africa.* — "\V. Milne continues his account 

 of these, describing three more new species of Macrotrachela, nine of 

 Hahrotrocha, the new genus Otostephanos with two new species, and new 

 species of PJeuretra, Scepanotrocha, Mniohia, and Adineta. The new 

 genus Otostephanos has a ring of fair thickness round the corona, with 

 short breaks dorsally and ventrally. Whether this ring is real, as Milne 

 believes, or a ghost, as Bryce believes, its appearance is distinctive. The 

 name llonoceros, used in a previous paper, is pre-occupied, and is re- 

 placed by Henoceros. 



Ecbinoderma. 



Division of Sea-urchin Ova.t — Ralph S. Lillie has studied the 

 influence of hypotonic sea-water on the divisions of the egg of Arhacia. 

 Sea-water of a dilution sufficient to cytolyse all unfertilized eggs in half 

 an hour or less, causes osmotic swelling, but not cytolysis, in undivided 

 fertilized eggs (up to a few minutes before cleavage begins). At or 

 about the time of formation of the cleavage-furrow a marked decline 

 takes place in the resistance of the &^^ to hypotony, and cytolysis is 

 then rapid and complete. When the cleavage-furrow is fully formed 

 the original resistance returns. A similar reversible decline of resistance 

 takes place at the second and third cleavage, and is probably general for 

 mitotic cell-division. 



The minimum of resistance is found during the formation of the 

 furrow. Both the decline and the return of the resistance are rapid, 

 the greater part of each phase occupying four to five minutes at 22° C. 

 Some increase of susceptibility is apparent ten or twelve minutes before 

 the first appearance of the furrow. 



A decrease in the coherence or extensibility of the plasma membrane 

 at the time of cytoplasmic division is thus indicated. The earlier 

 observations of Lyon and other investigators have shown that an 

 increase of susceptibility to poisons, heat, and other injurious conditions, 

 together with an increased output of CO.,, takes place at this time, i.e. 

 simultaneously with this change in the membrane. The above facts 

 constitute additional evidence that an intimate connexion exists 

 between the general physiological condition of the q^^ and the physical 

 state of the plasma-membrane. 



The above change in the membrane is probably associated with an 

 increased permeability to water-soluble substances and a decreased 

 electrical polarization ; this latter change, according to the law of 

 electrocapillarity, involves increased surface tension. From the analogy 

 with the general stimulation-process, it seems also probable that the 

 change of polarization acts upon the conditions within the dividing cell 

 (oxidations, etc.) in a manner analogous to the similar change in 

 electrical stimulation in general. 



&^ 



* Journ. Quekett Micr. Club. xiii. (1916) pp. 149-84 (5 pis.). 

 t Journ. Exper. Zool., xxi. (1916) pp. 369-402. 



