6 Cutler, Parthenogenesis in Animals 



At about the same time Whitney was working at this problem 

 with five species of Rotifers : Hydatina senta, Brachionus pala, 

 Diaschiza iterea, Diglena catellena, and Pedalion mirium. If the 

 parthenogenetic females of Hydatina senta, Diaschiza iterea, and 

 Diglena catellena are fed on a Polytoma diet female-producing 

 daughters are exclusively developed; if these females are, how- 

 ever, transferred to a diet of Chlamydomonas they produced 80 per 

 cent, or more male-producing 'daughters. A scanty diet of green 

 flagellates favoured the production of female-producing females 

 in the other two species of Rotifers, while a superabundance of 

 the same diet caused high male production. 



From these experiments the conclusion was drawn that the 

 change of diet was the external agent for the production jof males 

 and females. However, as Shull has remarked, the increase in 

 the production of males when the diet was a Chlamydomonas one 

 may be explained by the increased oxygen which is produced by 

 photosynthesis; this assumption is also' borne out by the experi- 

 ments of feeding scantily br abundantly with cultures of green 

 flagellates. This explanation is, however, not entirely satisfactory, 

 for when green algae, which were too large to be eaten, but 

 which would produce oxygen, were put into the water in which 

 the animals were living, the result was negative. Thuja it is prob- 

 able that nutrition has some effect, but to what extent is not defi- 

 nitely ascertained. 



Internal Factors as Operative Agents. 



As already mentioned, Punnett came to the conclusion that the 

 whole problem could be solved on the basis of heredity, and that 

 internal agents and not external ones were the operative factors. 

 This conclusion was tested by Shull. Two pure lines were 

 obtained, one from Baltimore, the other from New York; the 

 external conditions were as far as possible rigorously kept 

 constant. From the former the .proportion of males hatched was 

 ill per cent., from the latter 18-5 per cent., and these propor- 

 tions were found to be constant. Males and females from the 

 two pure lines were crossed, and the resulting lines had in every 

 case a higher proportion of male-producing females. On cross- 

 ing one of these hybrids back with either of the original parents, 

 the offspring gave rise to Jpfurfei lines with a proportion of males 

 intermediate between those of the two parent lines. Further, it 

 was shown that with long-continued parthenogenesis the per- 

 centage of male production was reduced. Punnett's conclusion 

 was not, however, entirely justified, for it was possible to alter 

 the constant ratio of male-producing females by changing the 

 external conditions. Thus the F 1 generation, which yielded more 

 male-producing females than its parents, was caused to produce 



