LOSS OF THE ECTODERM.OF AYDRA VIRIDIS 
IN THE LIGHT OF A PROJECTION 
MICROSCOPE. 
WILLIAM L. TOWER. 
WHILE working with a projection microscope in December, 
1897, I placed a living Hydra viridis in a small stage aquarium 
and projected its image upon a screen. The response of the 
hydra to this stimulus was startling. 
My apparatus consisted of an alternating current arc-lamp of 
fifty-two volt, twelve ampere capacity. The light was taken by 
a pair of four-and-one-half-inch condensers, and was passed by 
them through an alum cell to remove the heat rays, then through 
a bi-convex condensing lens, an Abbé condenser, and finally 
through the object into the objective. 
Projected in this manner, an unexpected sight was visible on 
the screen. The ectodermal cells, either singly or in groups, 
were seen to leave the animal, float free in the liquid, and grad- 
ually sink out of sight, leaving the hydra composed solely of 
endoderm, and the thin layer of mesodermal tissue. The cells 
were not distorted in any way, but retained their normal shape, 
acting as if they had been separated by some delicate and effect- 
ive disassociation method. This result, and the fact that it 
could not be due to heat, as will appear later, led to the further 
investigation of the phenomenon. 
The material had been collected in October, and kept in the 
laboratory, where it had multiplied to a considerable extent. The 
hydra were removed one at a time with a pipette, and placed in 
the stage aquarium ordinarily used for such purposes, where 
they were left from twelve to eighteen hours, during which time 
they fully recovered from being handled. They were then care- 
fully placed upon the stage of the projection microscope. By 
using sufficient care it was possible to bring the animal at once 
into the field in an unstimulated condition and fully expanded. 
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