260 Merkel Henry 'Jacobs 



the desiccation serving to start the development of the resting germ 

 cells ? Analogous cases are known in plants. Many seeds are 

 known to be incapable of germination until they have been dried 

 for a certain length of time. Drying is said to start into activity 

 the resting buds of trees. In Sphaerella lacustris, which is always 

 found associated with Philodina, microzooids are always pro- 

 duced in large numbers after a period of desiccation. Further- 

 more, in Loeb's experiments on artificial parthenogenesis, the 

 extraction of water from the eggs by hypertonic solutions fur- 

 nished a very effectual means of starting developmental processes. 

 Loeb has shown that the important factor in this case is the actual 

 loss of water rather than its reabsorption when the eggs are again 

 placed in normal sea-water. There seems to be no doubt, there- 

 fore, that under certain conditions loss of water may be an active 

 agent in starting processes of growth and development in resting 

 cells. 



It is interesting to note, although the fact may have no signifi- 

 cance, that the group of rotifers to which Philodina belongs, which 

 are the only ones of the rotifers normally subjected to frequent 

 periods of desiccation, at the same time are the only ones in which 

 sexual reproduction has been entirely lost. The thought suggests 

 itself that in the presence of such great changes in the external 

 conditions the stimulus furnished by the union of the sex cells 

 of different individuals is not so necessary as in most other 

 animals. On the other hand, it must be noted that in the aphids 

 parthenogenesis is said to continue as long as the external 

 conditions remain uniform, males being produced only when 

 there is a change in these conditions. Be this as it may, the fact 

 remains that desiccation in certain rotifers is able to bring on a 

 period of reproduction. Whether or not this is the only factor 

 involved it undoubtedly is an important one and one which may 

 be found to have a wider significance than we now suspect. 



VIII SUMMARY 



I Rotifers of all ages may recover all of their normal activities 

 after an extended period of desiccation. 



