THE PYCNOGONIDS. 9 



nucleus of the smaller segment — the micromere — divided first 

 and then the segment itself into equal parts. This was followed 

 five minutes later by the division of the nuclei of the large seg- 

 ment — the macromere — and then the macromere itself. 



In Fig. B an egg is shown as seen from above after this second 

 division, which is indicated by 2 — 2. The planes of segmentation 

 of the micromere and macromere are drawn in this figure as 

 though they coincided, but this is not the rule. The plane of 

 segmentation of the micromere may be turned at as much as 30° 

 (or exceptionally almost 45°) to that of the macromeres. I think 

 it should nevertheless be considered as one and the same plane 

 and one and the same division, in spite of individual variations. 



The segments again flatten together, and after an interval of 

 about an hour the third division commences. The plane of 

 division is at right angles to the last and is shown by Fig. C, in 

 which the egg is seen from above. Again the planes of micro- 

 mere and macromere do not quite coincide ; the numbers 2 — 2 

 indicate the second division and 3 — 3 give the plane of the 

 present (third) division. This plane is a zigzag line lying 

 between 3 — 3. If we examine the opposite pole, the lower, of 

 an egg at this stage, we find the large macromeres come together, 

 as shown in D, although there are many differences of detail in 

 this respect. So far the planes of division of micromere and 

 macromere have been supposed to coincide or to be referred to the 

 same division plane, but after this stage is reached (8 segments) 

 the planes cannot be considered identical. About an hour after 

 the last division the fourth rhythm comes on. This division plane 

 is shown for the macromeres in side view in figure E, by the line 

 4 — 4, and is seen to be parallel to the first plane of segmenta- 

 tion, 1 — 1. The four macromeres become eight. About the 

 same time — maybe five minutes before or after — the micromeres 

 divide. In Fig. E they are drawn after division has taken place, 

 and Fig. F shows how these cells are divided, the curved lines 

 indicating twin cells. As we see from the very nature of the 

 division of the macromeres, it is impossible to have their plane 

 of division to correspond with that of the micromeres, and the 

 division lines of the latter lie on a plane nearly at right angles 

 to the plane of division of the micromeres. We have now eight 

 macromeres and eight micromeres. After a resting period of 

 about an hour the fifth rhythm begins. 



