PEOGRESS OF MICROSCOPICAL SCIENCE. 81 



this experiment consists in placing the upper piece so that its cilia 

 vibrate at right angles to those of the lower. In this case, while the 

 lower cilia tend to move the upper piece from side to side, those of 

 the upper tend to move this lengthwise of the lower. The direction 

 which the upper piece takes is a resultant one, viz. intermediate 

 between the two. Then follow the experiments in air. Though the 

 tissues of the gills of Unios and Anodontas are quite soft and 

 incapable of resisting other than very light weights, they will never- 

 theless carry small disks of paper supporting a bristle, on the top of 

 which is a small pellet of cotton or a flag of tissue paper. In order 

 to show the flag more distinctly, a board painted black should be 

 nailed to the edge of the one on which the gill rests, to make a back- 

 gi'ound. With this precaution the experiment may be seen over a 

 large room. To mark the distance traversed, a pointer of white paper 

 should be set up on the board supporting the gill and at the 

 beginning of the experiment, the end of the pointer brought in contact 

 with the end of the flag on the gill. "When left to itself, the disk on the 

 gill with its flag at once begins to move to the opposite side, and 

 the flag is seen to recede from the pointer. The distance traversed 

 may be increased to several inches, by placing two or more gills side 

 by side, the free edge of the first slightly overlapping the cut edge of 

 the second, &c. The mucous membrane from the roof of the mouth 

 of frogs is much more solid than the gills of Unios, and the cilia 

 vibrate with much greater force. Different ciliated membranes exert 

 very different degrees of force, but he has found none better suited for 

 experiments than that just mentioned ; especially, when taken from 

 the mouth of the bull frog, which gives a large surface. It has the 

 advantage, too, of keeping up its activity for twenty-four hom'S or 

 more, after being detached from its natural connections, if only kept 

 cool and moist. For moistening it water answers sufficiently well, 

 but the serum of the blood of the frog is still better. The attention 

 of the writer was first called to the possibility of moving weights 

 much larger than was supj)osed possible by noticing the ease with 

 which a piece of skin which was accidentally placed upon the ciliated 

 membrane was swept off. By loading the piece of skin with weights 

 the mass moved was found to be unexpectedly large. The author has 

 devised a curious and interesting machine for demonstrating to a 

 large class the amount of work which can be done by these ciliated 

 surfaces. It is described in the article to which we have referred. 



The Embryology of Chrysopa and its Bearings on Classification. — 

 This is a most important and valuable paper, which was read before 

 the American Association by Dr. A. S. Packard, jun. At one of the 

 former meetings he dealt with the embryology of Diplax. Now he 

 has taken up Chrysopa. He says he did not observe the formation 

 of the blastoderm, but the blastodermic skin (" amnion") of Chrysopa 

 is of the same structure as in Caloj)teryx. At the posterior end of 

 the egg the round nucleated cells are crowded together in the same 

 way as in Calopteryx. The primitive band is of the same general 

 form, and floats in the yolk as in Calopteryx, but more as in Aspi- 

 diotus, though it rests more on the outside of the yolk than in those 



