144 



CHAS. B. WILSON. 



each blastomere retains a portion of the old ectosarc near its 

 centre, and this is surrounded by a border of newly formed 

 ectosarc, whose relative width increases as segmentation 

 advances. 



Finally, the persistence of the point of attachment of the 

 second polar body greatly facilitates the orientation of the 

 egg. As soon as the first groove is formed the blastomeres 

 become almost perfectly spherical. Most eggs having total 

 cleavage show a rounding of the blastomeres, but in this 

 Nemertean they are almost separated, so that the egg assumes 

 a dumb-bell shape, with a short handle eccentrically placed. 



As would be inferred, they are easily separated, with 

 results to be noticed later. 



Certain interesting phenomena precede the coming together 

 and flattening of the blastomeres. In this egg, as in that of 

 Echinoderms (5), filose activities similar to those of the polar 

 bodies arise on the surface of the blastomere. 



In fact, they are continuations of the same activity which 

 has just been spinning the connecting bands of protoplasm 

 and producing the first cleavage groove. Fastened at a 

 point near the opening of the groove are the polar bodies 

 (fig. 46). 



Close watching in a good light now reveals spin-threads 

 forming between the blastomeres. A small papilla appears 

 nearly opposite the attachment of the polar bodies, and 

 gradually increases until it becomes half as large as one of 

 them. 



At first filose threads radiate from this papilla in different 

 directions, but they gradually fuse into one thick thread, 

 extending obliquely across the groove to a point a little 

 below where the polar bodies are attached (fig. 47). Half a 

 minute later another spin-thread appeared below the first, 

 but no papilla could be seen at its origin, probably because 

 it was not in perspective. This thread divided when half- 

 way across, and the two ends were attached separately 

 (fig- 47). 



At the same time spinning activities began on the opposite 



