342 MESSES. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



changing the temperature at which the experiments were made. He was thus able 

 to settle at least part of the cause of the natural seasonal variation. Herbst (39) 

 induced a maternal inheritance by giving the eggs an impulse towards artificial 

 parthenogenesis before fertilization. Tennent (90) claimed that he was able to 

 influence the inheritance by changing the alkalinity of the waters in which the crosses 

 were made. 



Now, it will be noticed that the alterations in inheritance cited above were all 

 produced by changing the external conditions at the time of, or immediately after, 

 fertilization. We consider that the change of inheritance found at Plymouth caimot 

 be due to any such cause. It was not due to the temperature at which the 

 fertilizations were made, for the inheritance in a given year was the same in early 

 spring as in the middle of summer, and, moreover, the average laboratory 

 temperature of 1912 was not above or below that of previous years. Again, as 

 we have shown above, it was not due to any change in the alkalinity of the water 

 used in the experiments. 



Vernon (95) suggested that the reason why his hybrids resembled Strongylo- 

 centrotus in the spring and Splmrechimhs in the summer might be due to the relative 

 ripeness of the two parent forms at the two seasons, summer being the maximum 

 period of maturity for Splice rechinus and sjjring that for Strongylocentrotus. Any 

 such suggestion with regard to the Echinus hybrids at Plymouth is excluded by the 

 facts stated above, that the inheritance was the same at the beginning and the end 

 of the ripeness period of each species as it was at its maximum. 



As, therefore, no such changes in external conditions at the time of fertilization, or 

 after, can be made to account for the alteration in inheritance described in this paper, 

 we incline to the view that the physiological condition of the germ cells of at least 

 one of the parent species was affected by some alteration in the external environment 

 during the period of their growth. This opinion is supported by five facts, which at 

 the same time indicate that it was the female germ-cells of E. miliaris which were 

 affected : — • 



1. Whereas in the years 1909-11 pure-bred cultures of E. miliaris were the 

 easiest and quickest of all to rear, in 1912 it was only with great difficulty that the 

 late larvse of this species could be obtained. 



2. It was crosses in which E. miliaris was used as the female parent that showed 

 the changed inheritance. 



3. While in previous years cross-fertilizations with E. miliaris gave uniformly 

 80 or 90 per cent, segmentation, in 1912 it was rare to obtain as much as 20 per cent. 



4. In the only cross-fertilization of E. miliaris eggs which gave a high percentage 

 of fertilization in 1912, some of the hybrids had maternal characters. 



5. Different cultures from one given fertilization always gave the same type of 

 inheritance. 



In addition to these reasons, our contention gains further support from the fact 



