Organisms from Eggs 135 



sible to change the position of the nucleus and the 

 distribution of the pigment in the egg. It follows from 

 this that the nucleus and the pigment are suspended 

 in rather fluid material, the former in the centre, the 

 pigment at or near the surface. The position of the 

 nucleus determines the first plane of segmentation, 

 since the nuclear division precedes the division of the 

 cytoplasm of the egg and the plane of nuclear division 

 becomes also the plane of the division of the whole egg 

 — a point which need not be discussed here. It was 

 found, however, by Lyon and the subsequent investi- 

 gators that the place where the micromeres are formed 

 and where the intestine of the embryo later originates 

 is little influenced by the centrifuging of the egg. The 

 localization of this spot must therefore be determined 

 by a structure sufficiently solid not to be shifted by the 

 centrifugal force. The intestinal stratum in the egg 

 contains the forerunners of the tissues which secrete hy- 

 drolyzing enzymes, e. g., trypsin into the digestive tract. 

 When the surrounding solution is altered in consti- 

 tution or when the temperature is too high, the intestine 

 instead of growing into the hollow sphere grows outside, 

 we get an evagination instead of an invagination of the 

 intestine. Such larvae may live for a few days but they 

 cannot grow into a living organism. The forces which 

 make the intestine grow into the hollow sphere are 

 unknown ; it may possibly be only the difference between 

 the tension on the external and internal surfaces of the 



