64 THE BIOLOGICAL PROBLEM OF TO-DA Y 



double gastrulation, there may arise, for instance, 

 four instead of two primitive ears, eyes, and nasal 

 organs ; and these arise from cell - groups, the 

 choice of which is determined by their relation to 

 the position of the gastrula-invagination. 



From various other experiments, conducted so 

 as to distort the normal course of development, I 

 have obtained parallel results. 



Taking frogs' eggs immediately after fertilisation, 

 I compressed them strongly between parallel, 

 horizontally placed glass plates. I then inverted 

 them, so that the vegetative pole came to lie 

 uppermost. In spite of their unnatural relation to 

 gravity, they developed further, and became ab- 

 normal, quite unsymmetrical embryos. 



In another experiment, taking a triton's eggs 

 after they had divided into two spheres, I sur- 

 rounded them with a silk thread in the plane of 

 the first cleavage, and tightened the thread until 

 the embryo assumed the form of a sand-glass. The 

 deformity of the resulting larvse was very different, 

 and perhaps depended on the tightness of the con- 

 striction. Some became greatly elongated, and 

 had developed so that the thread surrounded the 

 dorsal nerve- cord. In other cases the dorsally- 

 placed organs arose only from one-half of the sand- 

 glass-shaped embryo, while the other half gave rise 

 to the ventral part of the body. In this case the 

 dorsal organs (nerve -tube and notochord) were 

 doubled over like a snare, the head and tail ends, 

 the mouth and the region of the anus, being bent 

 in at the position of the constricting thread. 



