262 Influence of environment on animals 



illustrated by the frog's eg^, 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 0. 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 Qgg 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. Wlien, 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 normal 

 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 Avhole embryo, which, of course, possessed only 

 half the size of an embryo originating from a whole egg. 



(h) Experiments on hydroids. 



A striking influence of gravitation can be observed in a hydroid, 

 Antenmdaria 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, i.e. its apex continues to grow 

 vertically upwards when we put it obliquely into the aquarium, 

 while the roots gi-ow vertically downwards. The writer observed 

 that when the stem is put horizontally into the water the short 



