228 



Mr. E. C. Punnett. 



[May 15, 



(Fam. 3) was isolated soon after being laid and placed in clean water without 

 a trace of Euglena. After hatching it was kept without food for six hours 

 and then well fed. It laid 14 eggs which all became thelytokous ? 's. One 

 of these was again starved for 23 hours and, on being supplied with food, 

 laid six eggs which all gave rise to thelytokous ? 's. Lastly, one of these six 

 was starved for 20 hours. Though well supplied with food at the end of that 

 time it experienced some difficulty in growing up and laid only two eggs 

 before it died. From both these eggs hatched out thelytokous ? 's. The 

 relation of these generations is shown in the accompanying scheme. In none 

 of the other similar experiments made with purely thelytokous strains did 

 any arrenotokous $ 's appear, neither was the proportion of J 1 's raised as the 

 result of starving families of Type B. 



Eeference was made above to an instance in which, owing to failure in the 

 Euglena supply, a decoction of cabbage was used as food. This happened 

 during the 54th and 55th generations of a strain of Type C (see Fam. 4). 

 The animals were poorly nourished and laid very few eggs. Nevertheless the 

 strain just managed to tide over the period of dearth, and on Euglena being 

 again forthcoming it went on for 17 further generations before the experiment 

 was stopped. During this time 134 individuals produced were proved to be 

 thelytokous and none were found to be arrenotokous. 



In the face of such facts as these it is difficult to entertain the opinion that 

 either temperature or nutrition have any influence in determining the 

 production of arrenotokous ? 's. 



The varying proportion of arrenotokous ? 's in different cultures of 

 Hydatina apparently depends upon the existence of ? 's of different zygotic 

 constitution, and the solution of the sex problem seems to lie in the deter- 

 mination of the unit-characters involved. The data for such a solution are at 

 present inadequate. Before it is possible to frame a theory we must have 

 further knowledge on the following points : — 



(a) The number of different types of ? that exist as judged by the criterion 

 of their relative output of thelytokous and arrenotokous ? 's, and the relation 

 of these types to one another. 



(I) The possible existence of zygotically distinct types of <$ 's and the types 

 of $ 's from which they respectively arise. 



With regard to the former point we have evidence for the existence of 

 (1) purely thelytokous ? 's, (2) of ? 's producing arrenotokous ? 's in the 

 proportion of about 7 in 16, (3) of strains of ? 's which produce but few 

 arrenotokous ? 's and in which the rather irregular numbers may be due to 

 the co-existence of two types at present indistinguishable owing to imper- 



