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THE AMERICAN NATURALIST [Vol. XLII 



evaporate they all die, presumably of starvation. It is 

 also interesting to note that some pools do not become 

 dry during the summer but the rotifer fauna changes 

 completely several times during the season. A small 

 pond in this vicinity was teeming with Asplanchna 

 brightwellii and Branchionus urceolaris during the 

 early part of July but by the middle of August not an 

 individual of either species could be found in it. Indi- 

 viduals of Pedalion and Polyarthra were very numerous 

 at this time but in the latter part of August not one 

 could be found. The pond was now teeming with 

 Branchionus angularis, B. pala and Triarthra longiseta 

 but no individual of the first four species named above 

 was present. 



In a case like this desiccation could play no part in the 

 preservation of the species and they could only be saved 

 by winter eggs. 



Some winter eggs of Asplanchna bright icellii which 

 were laid in June in artificial cultures were buried July 

 3 in an ice house upon a cake of ice where the temperature 

 was 1-2° C. On August 7 they were removed from the 

 ice house to ordinary room temperature and the old cul- 

 ture water replaced by fresh spring water. At the end 

 of forty hours several young Asplanchna were swimming 

 about in the water. 



Many other winter eggs which were laid at the same 

 time as the above lot but which had remained in the labo- 

 ratory at room temperature in a bottle containing water, 

 taken from the culture jar in June were placed in fresh 

 spring water. In many cases within an hour the thick 

 outer egg membrane had cracked open and exposed about 

 a fourth of the thin inner membrane which surrounded 

 the embryo. The history of some of these eggs was fol- 

 lowed and it was found that they produced normal young 

 animals on the following day. The swelling and crack- 

 ing open of the thick outer membrane is obviously due to 

 the sudden great change of osmotic pressure which is 



