818 



SCIENCE. 



[Vol. II., No. 47. 



band of cells, the chorda-entoblast, entering into the 

 formation of the entoderniic wall, but re^^enibling 

 in character the ectodermal cells ; 2°, the develop- 

 ment of the mesoderm as a paired outgrowth from 

 the blastopore. In part second of his paper, Hertwig 

 reviews the published investigations on the embryol- 

 ogy of other classes of vertebrates. He accepts the 

 homology of the primitive streak in Amniola with 

 the blastopore. He is fairly successful in proving the 

 same relations of the germ-layers to exist in all i'er- 

 tebrates. He also discusses the various objections 

 advanced against the coelomtheorie, according to 

 which the mesoderm is an epithelial layer, bounding 

 the body-cavity. He draws from his observations 

 and argiuuenls the following conclusions: 1. The 

 mesoblast grows as a continuous mass from acknowl- 

 edged epithelial layers; 2. In all vertebrates there 

 early appears a fissure in the mesoderm, limited pa- 

 rietally and vi>cerally by epithelium, as can be espe- 

 cially well seen in elasmobranch embryos; 3. From 

 this epitheliinn are derived true epithelial membranes 

 in the adult, from which are developed the perito- 

 neum, kidneys, sexual glands, etc.; 4. The primitive 

 mode of ori'^in of the mesoderm is probably that de- 

 scribed by Kowalevsky and Hatschek in Ampbioxus, 

 — an invagination of an epithelial membrane (en- 

 toderm) ; 5. In the true vertebrates the mesoderm 

 grows out as a solid mass, in which the fissure ap- 

 pears later. This must be regarded as a secondary 

 modification, for we frequently find hollow organs 

 making their first appearance as solid anlag en ; e.g., 

 the central nervous system of teleosts, many sense- 

 organs, and most glands. These considerations lead 

 collectively to the final conclusion that the meso- 

 dennic plates are morphologically epithelial evagina- 

 tions homologous with those of the invt-rlebratcs. 



Charles Skdgwick Minot. 



ACOUSTIC ROTATION APPARATUS. 



In a recent number of the Zeitschrift fib- insfru- 

 mentenkunde, Dr. V. Dvorak gives an account of the 

 various forms of apparatus which 

 have been devised to show attrac- 

 tion or repulsion due to sound- 

 waves, or to gain a continuous 

 rotHtion. 



Such experiments require a 

 good volume of sound for suc- 

 cess. That this may be obtained, 

 not only should the tuning-fork 

 be in accord with the resonator- 

 box on which it is placed (the 

 most convenient form of sound- 

 ing-body for the purpose), but al- 

 so tlie elastic system, consisting 

 of tuning-fork and box, should be 

 capable of vibrating in unison 

 with the fork and the air in the 

 Fio. ]. resonator. The three sounds are 



called the fork, the air, and the 

 wood tone. In'order to get tlie last, the resonator 



Fig. 



should be stuffed with cotton-wool, and a piece of 

 cork put between the prongs of the fork ; then, by 

 rapping on llie top of the fork, the whole system 

 is vibrated very much as it would be by tlie up-and- 

 down motions of the lower part of the fork wlien 

 free. By cutting away the walls of the resonator 

 to make them thinner, the system may readily be 

 brought to the right pitch. In most of the resona- 

 tors in common use the wood tone is too low, owing 

 to the wood being already too thin. 



The fork used by Dr. Dvorak was G, having 392 

 vibrations per second. It weighed 205 grams. As a 

 resonator, an ordinary 

 pine box was used, 

 about 13.5 cm. long, 

 11 cm. broad, and 10.5 

 cm. high. In one side 

 a round hole was cut, 

 large enough to make 

 the air tone of the 

 right pitch. The wood 

 was S mm. tliick. 

 From the top and bot- 

 tom it was shaved off 

 for the purpose ex- 

 plained above. The 

 dimensions of the box 

 were entirely acciden- 

 tal, but proved to be 

 good. 



By using an electro- 

 magnet to keep the 

 fork in continuous vibration, the results are naturally 

 more sure. The form of magnet which has proved 

 satisfactory is shown in fig. 1. E is the magnet, with 

 a core made of iron plates. This magnet is placed 

 between the prongs of the fork, and is lield by the 

 wooden arm a c, to the lower end of which is fas- 

 tened the resonator K. At 

 b the arm is bound to a firm 

 support, so that the system 

 of folk and resonator is 

 perfectly free. 



The resonator-wheel (fig. 

 2) is the first form of ro- 

 tating apparatus described. 

 It consists, as shown in the 

 illustration, of four glass 

 resonators on the four arms 

 of a wheel. For a fork of 

 392 vibrations, the spheres 

 should be about 44 mm. in 

 diameter, with openings 4 

 mm. across. Rotation was 

 obtained with the fork 40 

 cm. away. 



As a modification of this 

 wheel, a rotating resonator 

 (fig. 3) may be made of a 

 flat cylindrical pasteboard bos, having a number of 

 side-openings, each ending in a short piece of tubing 

 of size to make the resonator respond to the fork. 

 When suspended by a silk thread, h, such a resonator 



Fig. 



