10 T. H. MORGAN. 



The nuclei of the macromeres of Fig. E show by their elonga- 

 tion how they are to divide, and the plane of division lies between 

 them or at right angles to the last division plane. Fig. G shows 

 the egg after division into eight micromeres, but the micromeres 

 had not as yet divided and remain eight in number. Soon, 

 however, they do divide, but no definite plane of division for 

 all of the segments was discovered. There are now sixteen 

 macromeres and sixteen micromeres. 



The next plane of segmentation of the macromeres — at the 

 sixth rhythm — comes in after another resting period of an hour. 

 This is shown by Fig. E at 6 — 6, 6 — 6. The two planes are at 

 right angles to the last two (5 — 5) and are parallel to the first 

 and fourth (1 — 1, 4 — 4). I did not see the sixteen micromeres 

 divide into thirty-two, nor could I trace them further. There 

 are thirty-two macromeres and sixteen micromeres, .or in all 

 forty-eight cells. After an hour's interval the macromeres lying 

 between the planes 1 — 1 and 4 — 4 in Fig. H were each seen to 

 have two nuclei, but this did not seem to be the case with the 

 macromeres of the lower pole. The egg had been under obser- 

 vation for mauy hours and did not develop further, so that this 

 last attempt at division in the upper macromeres may have no 

 special significance. 



Besides the general facts of segmentation as just given, several 

 interesting variations in the method of segmentation of the egg 

 were clearly made out. In the example given above the micro- 

 meres divided first (at the second segmentation), giving two 

 micromeres and one macromere. In two other observations this 

 was also true, but in three other cases the macromere divided 

 live minutes before the micromere. And again in the last case 

 the two macromeres divided into four before the two micromeres 

 divided into four, so that the greater amount of yolk of the 

 macromere did not seem altogether to retard segmentation. In 

 several later stages (thirty-two macromeres and sixteen micro- 

 meres) the rhythm of macromeres and micromeres given above 

 did not so closely correspond, but the micromeres seemed to 

 drop behind. 



If the accumulation of yolk has been a very recent event in the 

 egg of Pallene — and the variations in size may bear this inter- 

 pretation — these differences in the method of segmentation may 



