240 The Microscope. 



von Mohl, serves different purposes in the economy of the cell- 

 In general, we may say, that it is either simply protective or 

 supporting, as in the case with the cells composing the veget- 

 able fabric of higher plants, when it even becomes lignified. 



Since the researches of M. Schultze, many have denied the 

 existence of a membrane in those cells endowed with amosboid 

 movements, but we can easily find cells with an evident mem- 

 brane, which still possess this amoeboid movement in a remarka- 

 ble degree, for example the testicular cells ; and such may be 

 found abundantly in the Arthropoda and especially in the class 

 Myriapoda. 



In conclusion, I would say that this living membrane is not 

 that solid and differentiated structure commonly regarded as 

 cell-membrane. This would be just as incorrect as it would be 

 wrong to consider the cuticle of the skin of animals or the epi- 

 derm of plants as the true and real, active, living skin or layer 

 of the bark ; further we do not maintain that in all cases this 

 true membrane of the cell persists, but it is a fact not to be called 

 into question that often the reticulum of this membrane becomes 

 differentiated into plastin, keratin, elastin ; and that the enchy- 

 lema is often replaced by air or by cellulose, chondrin, gelatin, 

 conchiolin, chitin, etc., etc., while the membrane often becomes 

 incrusted with mineral substances either calcareous or siliceous.. 



The term membrane therefore is justified in all cases, and it 

 may be used in its proper sense quite as well as other terms, such 

 as membranous layer, limiting membrane, primordial utricle 

 and others which are synonymous. 



NOTES ON THE MICROSCOPE STAND AND ON SOME 

 OF ITS ACCESSORIES. 



AN AMATEUR. 



XIX. 



AIR AND OIL BUBBLES, THE BROWNIAN MOVEMENT, THIN GLASS. 



RATHER than|take up space by an attempted description of 

 air bubbles, I recommend the reader, if he be not already 

 familiar with them, to|follow the plan suggested by Prof S. H. 

 Gage in his " Notes on Microscopical Methods," and make bub- 

 bles for examination. He will then know what they are, how 

 they look and ^how to recognize them when they unexpectedly 



