147 



spindle directs one pole in all cases towards the protrusion so that one 

 of the first two nuclei passes toward or into the protruded protoplasm. 



B. 



Deiesch has studied carefully the position of the microraeres 

 in eggs that have been subjected to pressure. I attempted to extend 

 his experiments. First I placed under pressure eggs that had under 

 normal conditions reached the four cell stage. At this stage when 

 compression is applied nearly every egg of Arbacia Orientes itself with 

 the first and second planes of cleavage lying vertically. If these eggs 

 were rotated under very slight compression through 90 ° so that the 

 side of the egg was turned up, and then additional pressure were 

 applied , we should be in a position to determine whether the for- 

 mation of the micromeres is affected by pressure (after the 

 four cell stage). Their normal position would be then at one end of 

 the egg. The third cleavage in such eggs is always vertical i. e. 

 both in the vertical plane of pressure and also in the normal position 

 of the third cleavage plane. Several successful experiments showed 

 distinctly that under these conditions the micromeres did not appear 

 above or below, but at the side of the egg in their normal position. 

 Professor E. B. Wilson pointed out to me that this result was not 

 conclusive because after the second furrow has been formed the po- 

 sition of the micromeres may have then been determined. The follow- 

 ing experiment was then devised. If we start with a normally fer- 

 tilized but unsegmented egg and place it under compression, we know 

 from Prof. Whitman's observation that the micromeres ought to form 

 at the pole opposite to the nuclear pole. We also know that in the 

 compressed egg the first and second furrows will appear vertically. 

 Therefore if an egg be chosen in which the nucleus lies to one side 

 (i. e. not in the center of the egg above or below) we should expect 

 to find either that the micromeres will form at the pole opposite the 

 nuclei (as under normal conditions) or that the micromeres will form 

 above or below and not in their normal position. We are then able 

 to discover positively whether or not the position of the formation of 

 the micromeres is predetermined. Fig. 1 a— d shows the cleavage of 

 the normal egg and Fig. 2 a— d shows the cleavage of the compressed 

 egg. In Fig. 1 a — d the micromeres are seen to form at the pole oppo- 

 site to the nuclei of earlier stages and to lie at the crossing point 

 of the first and second furrows. Examining the compressed series 

 drawn in Fig. 2 a — d we find that the first furrow comes in between 

 the excentric first two nuclei. The second cleavage is also vertical 



