ORGANIZATION AND GROWTH 231 
When a transverse incision abe (Fig. 59) is made 
transversely in a Cerianthus membranaceus fairly near 
the oral pole, the tentacles 7, situated above the incision 
collapse like the wilted portions of the plant. When I made 
such a transverse incision in a Cerianthus the tentacles of 
which were growing actively, the tentacles 
above the incision ceased to grow, while the 
remaining tentacles went on.’ Cerianthus, 
however, could not be used for a detailed 
study of the dependence of animal growth 
upon cell turgor. Longitudinal growth can- 
not be measured accurately in contractile 
animals. Moreover, in the experiment on 
Cerianthus described above the amount of 
water absorbed by the organs cannot well 
be controlled experimentally. And, finally, 
the growth of Cerianthus is relatively slow. 
s I therefore chose a more suitable animal upon 
which to experiment, namely Tubularia, and 
a different method of varying the amount of 
water absorbed —the osmotic. I am obliged to Dr. Wort- 
mann for calling my attention to the plasmolytic experiments 
of de Vries. 
The stem of Tubularia is surrounded by a rigid periderm, 
and an increase in length can be measured as accurately in 
this animal as in the rigid stem of a plant. I changed the 
concentration of the sea-water through the addition of sodium 
chloride or fresh water. The temperature was always the 
same for all the animals in the same series of experiments; 
the amount of liquid in each vessel was always 300 c.c.; and 
the number of animals in each vessel was also nearly always 
the same. 
2. As is well known, Tubularia grows in length only when 
FIG. 59 
1See “‘ Heteromorphosis,” Part I, p. 155, 
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