INHERITANCE IN HYDATINA 87 



the reciprocals were quite easily reared. The results obtained 

 from Hydatina are stated in a preceding section of this paper. 

 When the unequal reciprocals were inbred, and reciprocally 

 crossed, it was found that all traces of the supposed influence 

 of the cytoplasm of the egg had disappeared. The inequality 

 lasted only during the egg stage, and ceased when the eggs gave 

 rise to parthenogenetic lines that could be compared. 



In this case it seems necessary to regard the cytoplasm as 

 part of the environment of the zygote. When new cytoplasm 

 had been manufactured under the influence of the combined 

 nuclei of the two lines, this cytoplasm was approximately equal 

 in the two reciprocal lines. 



SUMMARY 



The fertilized eggs of Hydatina senta, unlike the partheno- 

 genetic eggs, do not all hatch. The proportion hatching in var- 

 ious lots of eggs has varied from zero to seventy per cent. The 

 capacity for hatching I have called the 'viability' of the eggs. 

 Some of the factors apparently governing the viability of the 

 eggs are discussed in an early part of this paper. 



The viability of eggs produced by inbreeding males and females 

 of the same parthenogenetic line is characteristic of the line 

 producing them. One line produced eggs, none of which hatched; 

 another line produced eggs of which 5 per cent were viable; 

 another showed 40 per cent of viability, and so on. 



The viability of the eggs is inherited. When a line whose 

 inbred eggs possessed a viability of 5 per cent was reciprocally 

 crossed with another line with 45 per cent of viable eggs, the 

 recijyrocal hybrid lots of eggs were unequal in viability. Each recip- 

 rocal was mo7x nearly like the inbred eggs of the maternal line. 

 This result was obtained in two distinct pairs of reciprocal crosses. 



The age of fertilized eggs at hatching varies greatly. One 

 egg I have observed hatched in four days after laying; two weeks 

 is a more common age; while many eggs doubtless never hatch. 

 The degree of uniformity of the duration of the egg stage is a 

 characteristic of the line producing the eggs. The inbred eggs 

 of one line practically finished hatching (45 per cent were viable) 



