298 
GENETICS: K. S. LASHLEY 
Summary. 1. Emphasis has been placed upon the necessity of con- 
sidering the pole effect in the redetermination of wave-lengths in 
international units. 
2. The wave-lengths of these sensitive lines are not affected by a 
wide variation of density of the radiating vapor. 
3. Their wave-lengths are independent of changes in temperature 
over the range of our observations. 
4. For the lines considered the pole effect does not occur in vacuOy 
and in so far appears independent of electrical conditions. 
5. The observed pole effect does not vary with wave-length in the 
same way as pressure shift, and cannot be explained as a pure pressure 
effect unless the pressure changes in certain definite ways with the 
wave-length. 
1 ML Wilson Contr. No. 61; Astrophys. J. 36, 37 (1912). 
2 ML Wilson Contr. No. 58; Astrophys. J. 35, 10 (1912)= 
INHERITANCE IN THE ASEXUAL REPRODUCTION OF 
HYDRA VIRIDIS 
By K. S. Lashley 
ZOOLOGICAL LABORATORY, JOHNS HOPKINS UNIVERSITY 
Presented to the Academy, March 17, 1915 
Do heritable variations commonly occur among the offspring of a 
single individual multiplying asexually? May selection among such off- 
spring produce strains differing in hereditary characters? The inves- 
tigation here resumed is designed to contribute data toward the answer 
to these much debated questions. 
A number of specimens of Hydra viridis, taken at random from wild 
populations, gave rise by asexual reproduction to clones differing from 
one another in their average number of tentacles and in other charac- 
ters. As a test of whether such differences are the result of internal fac- 
tors or of environmental differences two clones were bred in large num- 
bers for a period of five months, during which time the members of the 
two clones were kept under environmental conditions as nearly as pos- 
sible the same. Each polyp was kept in a separate culture dish and food 
was distributed uniformly to all. The number of tentacles was recorded 
at the time when the polyps began their independent life after separa- 
tion from the parents (the initial number of tentacles), and records were 
kept, also, of changes in the number of tentacles of parents. 
The two clones kept under parallel conditions gave the following 
results. The average number of tentacles of 1353 members of one 
