262 Influence of environment on animals 
illustrated by the frog’s egg, which has two layers of very viscous 
protoplasm one of which is black and one white. The dark one 
occupies normally the upper position in the egg and may therefore be 
assumed to possess a smaller specific gravity than the white substance. 
When the egg is turned with the white pole upwards a tendency 
of the white protoplasm to flow down again manifests itself. It is, 
however, possible to prevent or retard this rotation of the highly 
viscous protoplasm, by compressing the eggs between horizontal 
glass plates. Such compression experiments may lead to rather 
interesting results, as O. Schultze first pointed out. Pflueger had 
already shown that the first plane of division in a fertilised frog’s 
egg is vertical and Roux established the fact that the first plane 
of division is identical with the plane of symmetry of the later embryo. 
Schultze found that if the frog’s egg is turned upside down at the 
time of its first division and kept in this abnormal position, through 
compression between two glass plates for about 20 hours, a small 
number of eggs may give rise to twins. It is possible, in this case, 
that the tendency of the black part of the egg to rotate upwards 
along the surface of the egg leads to a separation of its first cells, 
such a separation leading to the formation of twins. 
T. H. Morgan made an interesting additional observation. He 
destroyed one half of the egg after the first segmentation and found 
that the half which remained alive gave rise to only one half of an 
embryo, thus confirming an older observation of Roux. When, how- 
ever, Morgan put the egg upside down after the destruction of one of 
the first two cells, and compressed the eggs between two glass plates, 
the surviving half of the egg gave rise to a perfect embryo of half 
size (and not to a half embryo of normal size as before). Obviously 
in this case the tendency of the protoplasm to flow back to its nomnal 
position was partially successful and led to a partial or complete 
separation of the living from the dead half; whereby the former was 
enabled to form a whole embryo, which, of course, possessed only 
half the size of an embryo originating from a whole egg. 
(6) Eaperiments on hydroids, 
A striking influence of gravitation can be observed in a hydroid, 
Antennularia antennina, from the bay of Naples. This hydroid 
consists of a long straight main stem which grows vertically upwards 
and which has at regular intervals very fine and short bristle-like 
lateral branches, on the upper side of which the polyps grow. The 
main stem is negatively geotropic, ie. its apex continues to grow 
vertically upwards when we put it obliquely into the aquarium, 
while the roots grow vertically downwards. The writer observed 
that when the stem is put horizontally into the water the short 
