418 C. F. ROUSSELET ON PROALES WERNECKI. 



perature it is possible to impart at will to the embryo the sexual 

 character one may desire. It is probable, however, that there 

 are other factors at work, such as scarcity of food, drying up of 

 the water, and generally when the conditions become unfavour- 

 able to the life of the rotifers ; for it is a well-known fact that 

 males and resting eggs are produced at all seasons of the year, 

 including the winter when the temperature is low, though the 

 summer is no doubt the more favourable season for their 

 production. 



Professor Eothert has found both female and male eggs, and 

 also female and resting eggs of P. wemecki in the same Vaucheria 

 gall, but it does not necessarily follow that these were laid by 

 the same female, as I have seen two or three females in the same 

 gall. A more important observation of his is that resting eggs 

 undergo segmentation and partial development before the second 

 egg membrane is formed, and before the period of rest begins, so 

 that the resting eggs contain embryos in an advanced state of 

 development ; this is probably true of resting eggs of all rotifers. 



The Vaucheria I had received degenerated rather quickly, 

 after a fortnight, and the rotifers went with it; and no more 

 could be obtained, as the ditch had meanwhile dried up. My 

 opportunities of further study were therefore cut short. 



My friend Mr. F. It. Dixon-Nuttall soon informed me that he 

 also had found the male ; and the figures on the accompanying 

 plate are his drawings of the male and of a very young female 

 just hatched, showing the differences between the two. The 

 dorsal antenna is small, but can always be seen in both the male 

 and female, whilst the lateral antennae have so far escaped 

 detection. 



Size of male fully extended T f^ in. (149 /u). 

 „ „ young female „ T |^ in. (159 l x). 



„ „ adult female „ t Jq in. (195 /x), 



Explanation of Plate XIX. 



Fig. 1. Proales wemecki, young female, dorsal view. 



„ 2. „ ,, male, side view. 



„ 3. „ „ male, dorsal view. 



„ 4. ,, „ jaws of male. 



