Evening star., 1938-04-21

Some pinpoint-like bugs" gathered
from an Alexandria pond are shedding
light on one of the profoundest mysteries
of nature in the geological laboratory
of Johns Hopkins University in Balti-
more.
Barely visible except under a micro
scope hundreds of them live together
in a couple of drops of water. They are
Of a species of protozoa, or ons-celled
animals. known as Paramecium bursaria.
Ordinarily they reproduce by splitting
in two. Potentially, at least, they are
immortal. The splitting process might
60 on for as many generations as could
be counted.
In most multi-cellular forms of animal
life reproduction is more complicated
and on the whole, more efficient process
Two individuals unite to produce a
third. Eventually the individuals them
selves die and their substance is split
up into its original elements. This gives
the possibility of variation through in
heritance, in various mixings, Of the
hereditary characters of both parents
Hence comes the possibility OF evolution
and the building up of the many
branched tree of life. Both parts of
split protozoa are exactly alike, and so
on forever while the family line lasts.
But the paramecium stands on the
threshold OF the more advanced sys-
tenn. Occasionally, 1t has been observed,
two paramecia coalesce before splitting,
some of the substance Of each entering
into the resulting new animals. Some
months ago Johns Hopkins zoologist
made the discovery. in another para
-.
a matter Of pure chance between any
two individuals which morphologically
cannot be distinguished from each other.
Ordinarily the tiny creatures live in
colonies of several hundred individuals.
1t was found that individuals from the
same colony would not mate. Although
physical differences could not be dis
tinguished, it was concluded that here
was the beginning in life of sexual diff
ferentiation, perhaps basically chemical
in nature. There were not two sexes, but
two sex reaction types from which
morphologically differentiated sexes
might be expected to arise at higher
steps in the ladder of life.
Now DR.. H. s. Jennings, Johns Hop
kinS professor of zoology, has carried
the investigation little further. The
type of parameciae he used apparently
were just a step more primitive than
those which were used in the previous
experiments. They consisted of two
colonies taken from the Alexandria
pond. Each of the colonies, DR.. Jen-
nings found, represented different sex
reaction type. They were bred together.
They would not do 50, it is interesting
to note, so long as they were well fed.
With plenty to eat, individuals kept
splitting without mating. Only when
subjected to semi-starvation did the
mating reactions take place.
types also showed the differentiation in
to two types. They also proved excep-
tionally delicate, perhaps due to slight
variations from the standardized form
that had become ideally adapted to Its
environment. The majority of them
died before the lines of descent could
be carried much further. DR.. Jennings
succeeded, however, in establishing 26
family lines.
Then he was astounded to find not
two, but four, sex reaction types. Four
groups were established, each of which
would mate with all the other three, but
not with itself. 1t was equivalent to
four infra-sexes. Then DR.. Jennings
proceeded to make the picture still
more complicated. He gathered % other
paramecium colonies from . pond near
Baltimore and proceeded to breed them
IA the same way, both Yith each other