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NOTES AND MEMOKANDA. 
Tremhley^s Expermients on Turning a Hydra Inside Out , — Professor 
Th. W. Engelmann, of Utrecht, communicates the following to the 
‘Zoologischer Anzeiger ’ : — Among Trombley’s many remarkable state- 
ments concerning the vital properties of Hydra none has awakened 
more surprise than that the animal after being turned inside out 
continues to live, and is able to take nourishment, to digest, &c. 
Whilst the statement, if it be correct, would involve the upsetting of 
many of the most important and apparently best founded morpho- 
logical and physiological doctrines, yet, on the other hand, Trembley 
is noted as a most exact and credible observer, and it therefore seemed 
to me well worth while to repeat his experiments even at the risk of 
succeeding no better than honest Rosel, the only one I know of who 
appears to have repeated them carefully. 
I have therefore every year since 1873 tried Trembley ’s experiment, 
in doing which I narrowly followed his directions. An animal of 
suitable size and a hog’s bristle of proper size and shape, with a 
pretty steady hand, are all that is wanted. The experiment is by 
no means difficult. The results, however, were without exception 
unfavourable to Trombley’s assertion. 
The turned body of the polyp when it did not soon resume its 
normal position always perished within a short time, the cells, and 
first of all those of the entoderm, swelled very much, gradually 
loosened themselves from their connection, and were found after a 
day or two like a small white cloud at the bottom of the glass beneath 
or beside the remainder of the polyp, only the anterior part of the 
body containing the thread cells, and which consequently cannot bo 
quite turned, remained alive in many cases, and (after the turned part 
had died and been pushed off) developed a new body from behind, 
sometimes in a slanting direction. Under the microscope this showed 
the ectoderm outside, and inside the entoderm, with their known 
histological structure. In many cases the whole polyp died. 
That the conditions were not unfavourable for the success of the 
experiment, was evident from the fact that under precisely similar 
circumstances extremely small pieces of the tentacles cut off were 
frequently observed to develop into perfect five-armed Polypi, and 
HydraB, which had been slit longitudinally, to grow together, &c. 
For my first experiment I had taken animals from slow-running 
water. As Trembley’s polypi were mostly derived from stagnant 
waters, I experimented afterwards with such specimens, but with the 
same negative results. There can therefore be no other conclusion 
come to than that this otherwise so careful observer has for once been 
deceived. Certainly if we re-peruse many of his minute descriptions 
we should like to think that we had done him injustice in this 
assumption, otherwise we are driven to the further conclusion that he 
sometimes describes things most minutely of which in reality he has 
seen the least.* 
VOL. I. 
* ‘ Zoologischer Anzeiger,’ vol. i. p. 77. 
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