SEX-DETERMINATION IN ASPLANCHNA 249 



It is plain that Nussbaum's varying results are in general closely 

 parallel to our own variable results with our first general mass 

 cultures, obtained before a successful feeding technique and a 

 successful analysis by means of isolation cultures had fully indi- 

 cated the probable solution. In so far as the exact conditions 

 of Nussbaum's experiments are given they harmonize fully with 

 our own results, while Nussbaum's main conclusions show the 

 strong impression made upon his mind by the copious male pro- 

 duction which was, by him, at least frequently observed to follow 

 starvation. 



In the work of Punnett which follows upon that of Nussbaum, 

 four pedigree series of Hydatina were carried through twenty-two 

 to seventy-two generations. These series yielded varying num- 

 bers of male producers, from which fact, Punnett concludes that 

 they belong to 'strains' possessing differing inherent capacity for 

 male production. He admits that these different 'zygotic consti- 

 tutions' may perhaps be modified by external conditions, though 

 he himself did not determine the cause of such modification. In 

 one instance he did record the production of a purely parthe- 

 nogenetic female-producing ('thelytokous') strain from a slightly 

 male-producing ('arrenotokous)' strain. In one experiment the 

 effort was made to produce male production by starvation of 

 young females, after hatching, from a line which had produced 

 no males. The result was wholly negative. It is plain that 

 these results of Punnett offer no difficulties to our interpretation. 

 His lines showing no male production and male production may 

 well have been parallel to our lines of saccate and humped As- 

 planchna respectively. They may have been merely upon differ- 

 ent physiological levels. That his starvation experiment failed 

 was, in the light of our experiments inevitable, in that it was be- 

 gun with individuals undoubtedly upon too low a level to be 

 capable of male production. 



UnifoiTn feeding was used throughout Punnett's experiments 

 with the exception of a single instance of a substitution of an 

 all but fatal diet in the case of a strain already of low potential. 

 The one instance where a strain dropped from low male produc- 



