216 The Mechanistic Conception of Life 



of different specific gravity, so that a change in the position of 

 the cell or the organ may give results which can be traced to a 

 change in the position of the two substances. This is very 

 nicely 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 upward a tendency of the white protoplasm to flow 

 dowTi 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. Pfliiger had already shown 

 that the first plane of division in a fertilized 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 do\Mi at the 

 time of its first division and kept in this abnormal position, 

 through compression between two glass plates for about twenty 

 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 upward 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, however, Morgan put the egg upside do^\^l 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 



