92 John H. Gerould, 



near the active pole. While the two nuclei are coming- more closely 

 into contact, the two asters become equalized, and the first cleavag-e 

 spindle, with twenty chromatic filaments about its equator, is pre- 

 sently established. 



5. Segmentation of the Egg. 



The fact that the character of the cleavag-e in the Sipunculids 

 has been hitherto unknown, except for the few observations of 

 Selenka (1875), which can be relied upon as accurate only as fai" 

 as the four-cell stage, has stimulated nie to pursue this part of the 

 subject in the face of certain difficulties, chief among which is that 

 of orientation. This, I found, can be managed by beginning obser- 

 vations upon the living egg in the four-cell stage, at which precision 

 is possible, and by making camera drawings, at short intervals, of 

 the changes which ensue in a particular quadrant or group of cells. 

 In the case of the earlier stages, I have repeated these observations 

 many times, and supplemented them by the study of preparations 

 of unstained eggs, mounted in glycerine. 



Convenience in comparing one organism with another is of 

 prime importance in nomenclature, and every tendency towards 

 uniformity in this matter should be fostered. Hence I shall employ 

 Conklin's modification of Wilson's plan, not only because it is con- 

 venient and easily followed, but because the general scheme has been 

 used by a large and rapidly increasing number of observers. Follow- 

 ing Mead (1897), Child (1900), Teeadwell (1901), Robert (1902), 

 Turkey (1903) and others. I shall give each "macromere" a coeffi- 

 cient corresponding to that of the "micromere" of the same gene- 

 ration. 



The terms "macromere" and "micromere" have become so widely 

 current, and are so readily understood to mean respectively the 

 quartet at the vegetative pole and the quartets Avhich arise succes- 

 sively from it, that I shall use them in this way in reference to 

 PJiascoJosoma, although their literal meanings do not in general 

 correspond to the actual sizes of the cells designated by them. 



The active pole of the egg, indicated by the position of the 

 polar bodies, becomes the middle of the apical plate of the 

 trochophore, and hence I shall refer to it as anterior. Of the 

 blastomeres of the 4-cell stage, A and B are, respectively, the left 

 and right ventral quadrants of the trochophore ; C and D, the right 

 and left dorsal. These designations of the quadrants, however, only 



I 



