2 THE AMERICAN MONTHLY [Jan., 



days a single individual may grow into millions and bil- 

 lions. Such water generally has a peculiar odor which 

 makes one hesitate to drink it. Obviously, it would be 

 a great mistake to take this water as a sample of that 

 which we drink. 



In the second place, earth and plants put into the tanks 

 contain many eggs of various kinds which likewise soon 

 develop and increase largely. 



Thirdly, there is the external air always full of dust 

 and germs. Currents of air entering the Conservatory 

 through the smallest crevices or openings bring in many 

 germs, which fall into the tanks and there develop. Tyn- 

 dall's illustration of floating matter in the air is familiar 

 to many. Indeed, in view of this, the wonder seems to 

 be that animalcules are not far more numerous than we 

 find them. It must be borne in mind, though, that all of 

 them have many enemies to check their too rapid increase 

 or are often placed where conditions do not favor them. 



It now remains to treat in detail some few of the 

 many interesting objects to be found in these tanks. 



On the under side of the leaves, or on the stems of 

 pond lilies, snail eggs are common.' These are very 

 transparent and provide one of the best means for watch- 

 ing the development of the snail. In fresh-laid eggs 

 we see a little dark colored spot on the central part of 

 each egg. Next we observe a division or segmentation 

 of this small mass, and thereafter it gradually increases 

 in bulk and different organs come into view. Finally, 

 we have a fully formed snail, shell and all, almost com- 

 pletely filling the egg. Then the animal by tugging and 

 pushing makes a rent in the thin egg case, and, emerg- 

 ing, immediately crawls about in search of food. Snails 

 it may be added, destroy large numbers of microscopical 

 forms. One may observe the track of a snail on the 

 under side of leaves or on the sides of glass jars. It is 

 very much like that left by the scythe of a mower. The 



