INHERITANCE IN HYDATINA 51 



of this solvent may account for differences in viability in differ- 

 ent lots of eggs, 



2. Muscular power of the embryo probably detennines the 

 hatching in part. Shells that have been made quite thin by 

 the solvent sometimes remain unbroken, owing to weakness of 

 the young rotifers. 



3. Some of the eggs classed as fertilized may not really be 

 fertilized, and so incapable of development, or else incapable of 

 producing an embryo that can escape from the shell. Several 

 years ago I obtained, by mating a female with an old male, a 

 family consisting in part of fertilized eggs, in part of male eggs. 

 This is the only case of the kind ever recorded. The fertilized 

 eggs were laid first, and one of them hatched, producing a female. 

 They were followed by a number of smaller eggs, all of which 

 had shells somewhat thicker than parthenogenetic eggs usually 

 have, but not so thick as fertilized eggs. Some of these small 

 eggs hatched, though the embryo remained in the egg stage 

 longer than the usual twelve to fourteen hours common to par- 

 thenogenetic eggs. A number of the small eggs did not hatch, 

 which is uncommon in parthenogenetic eggs. I believe this fail- 

 ure to hatch in some, and delay in hatching in others, to be due 

 to the thicker shells. In some way impregnation of the female 

 seemed to stimulate the production of thicker shells, even on 

 the eggs that were not fertilized. May this not be the case 

 frequently? Small eggs, sometimes not any larger than male 

 eggs, were not uncommonly found in these experiments; but 

 they had thick shells bearing the usual pilose projections, and 

 were classed as fertilized eggs. If these were unfertilized, any- 

 thing that caused a variation in the number of them produced 

 might affect the proportion of eggs that hatched. 



4. Environmental conditions no doubt play an important role 

 in the hatching of the fertilized eggs. There are plain indica- 

 tions that this is the case, though in what way or to what extent 

 has been only partially worked out. 



Which, if any, of the first three of these factors is responsible 

 for the variations in the \'iabiUty of the eggs that were the sub- 

 ject of these experiments is not known. 



