SCIENTIFIC SUMMARY. 
Ill 
adherence of the armature after the cessation of the current ; but this is not 
the case, since no cohesion is manifested even under a greater pressure than 
that caused by magnetisation. Moreover, a magnetic needle placed in the 
vicinity of one of the polar surfaces shows a marked deviation, which dis- 
appears as soon as the keeper has been tom away. But what is particularly 
worthy of notice is the persistence of this magnetic condensation. An 
electro-magnet has been left for twenty days, and at the end of this time 
the keeper still supported 50 kilos, without becoming detached. 
ZOOLOGY AND COMPARATIVE ANATOMY. 
Philosophical Experiments on the Auditory Apparatus of the Mosquito . — 
Professor A. M. Mayer has published in the u American Naturalist ” [Oct. 
1874] the results of a great number of most philosophically conducted 
experiments on this insect, and, as his paper is of some length, we must 
refer our readers to it as being a most valuable scientific record. He states — 
after recording some of his experiments, conducted with tuning-forks — that 
he thinks that it is to be regretted that Konig did not adhere to the form of 
fork, with inclined prongs, as formerly made by Marloye ; for with such forks 
one can always reproduce the same initial intensity of vibration by sepa- 
rating the prongs by means of the same cylindrical rod which is drawn 
between them. Experiments revealed a fibril tuned to such perfect unison 
with Ut 3 that it vibrated through eighteen divisions of the micrometer, or 
T5mm., while its amplitude of vibration was only three divisions when Ut 4 
was sounded. Other fibrils responded to other notes, so that he infers from 
his experiments on about a dozen mosquitoes, that their fibrils are tuned to 
sounds extending through the middle and next higher octave of the piano. 
The Development of the Ova and the Testicles in the Hydractinice. — M. Van 
Beneden has recently published an exceedingly valuable paper on the above. 
The following are the conclusions to which he is led by his researches : In 
the Hydractinire, 1. The eggs are developed exclusively from the epithelial 
cellules of the endoderm. They remain, up to the time of their maturity, 
surrounded by the elements of the endoderm. 2. The testicles and sperma- 
tozoa are developed from the ectoderm. This organ results from the pro- 
gressive transformation of a primitive cellular fold formed by invagination. 
3. There exists in the female sporosacs a rudiment of the testicular organ; 
in the male sporosacs a rudiment of an ovary. The sporosacs are then 
morphologically hermaphrodites Fecundation consists in the union 
of an egg, a product of the endoderm, with a certain number of spermatozoa, 
products of the ectoderm. This act has no other end than to unite chemical 
elements of opposite polarity, which, after having been united an instant in 
the egg, separate again ; for in most animals, those in which the division of 
the vitellus into two occurs, the elements from which the ectoderm are 
formed are already separated from those which are to form the internal layer 
of the embryo. 
The Development of the Trachece in Flies. — An excellent memoir on this 
subject is that of Herr Weismann, which has been recently translated 
