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DEVELOPMENT OF THE EMBRYO.

At a recent meeting of the Physical Society, Berlin, Prof.  Preyer spoke on reflexes in the embryo.  His researches extended over many classes of animals.  As representing mammals, guinea pigs were chiefly used; and for reptiles, snakes; while in addition the embryos of fishes, frogs, mollusks, and other lower animals were also employed.  But of all animals birds are most suitable for embryological observations, inasmuch as with due precautions the development of one and the same individual can be followed for a considerable time.  Birds’ eggs can be incubated in a warm chamber, and by removing a portion of the shell and replacing it by an unbroken piece from another egg, it becomes possible to follow the daily development of the chick and to experiment upon it.  As early as the ninetieth hour of incubation, spontaneous “impulsive” movements may be observed, taking place apparently without any external stimulus as a cause, and at a time when no muscles or nerves have as yet been developed.  After the occurrence of these spontaneous movements, and at the earliest on the fifth day of incubation, movements are observed to result from the application of mechanical, chemical, and electrical stimuli.  In order to observe these the eggs must be allowed to cool down until all spontaneous movements have ceased.  From the tenth to the thirteenth day more complicated and reflex actions occur on the application of stimuli, as, for instance, movements of the eyelids, beak, and limbs; and if the stimuli are strong, reflex respiratory movements.  These reflexes make their appearance before any ganglia have become differentiated.  Prof.  Preyer considered himself justified in concluding from this that ganglia are not essential for the liberation of reflex actions.  He intends, on some future occasion, to give a more detailed account of these experiments, and of the conclusions which may be drawn from them.  In the discussion which ensued the conclusions of the speaker were contested from many sides.

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IRIDESCENT CRYSTALS.[1]

   [Footnote 1:  Abstract of the Friday evening lecture delivered by
   Lord Rayleigh, F.R.S., at the Royal Institution, on April 12,
   1889.]

By LORD RAYLEIGH.

The principal subject of the lecture is the peculiar colored reflection observed in certain specimens of chlorate of potash.  Reflection implies a high degree of discontinuity.  In some cases, as in decomposed glass, and probably in opals, the discontinuity is due to the interposition of layers of air; but, as was proved by Stokes, in the case of chlorate crystals the discontinuity is that known as twinning.  The seat of the color is a very thin layer in the interior of the crystal and parallel to its faces.