154 University of California Publications in Zoology. [Vol. t; 



I have not been able to determine certainly whether in this 

 species segmentation begins at the sides and extends around as 

 Brooks describes. The infolding is farther advanced on the 

 neural side of the chain than on the side of the genital rod. 

 so the infolding either begins at the sides and works more rapidly 

 toward the neural tube or it begins at the neural tube and 

 extends around, reaching the ectoderm next the genital rod last. 



Near the base of the stolon, the genital cells are grouped 

 together irregularly, but passing distally these cells come to lie 



Pig. 8.— Surface view of stolon, showing point where segmentation be- 

 gins in chain of S. fusiformis-runcinata. 



more and more in a single row. Where the segmentation is as 

 plainly marked as in fig. 7. there is approximately one cell for 

 each segment. As it is usual in this species to find but a solitary 

 embryo in the aggregate individual, the final distribution of ova 

 in the chain probably gives one to each segment. 



An examination of whole mounts of chains showing the most 

 recently segmented portion next to the stolon, shows that a break 

 in the regularity of the progress of segmentation sometimes 

 occurs (Fig. 8). The constrictions marking off the segments of 

 the stolon almost or quite disappear for a short distance and 

 then reappear beyond, they being again gradually lost as one 

 approaches the rout of the stolon. Some chains do not seem to 

 have this break in continuity of the segmentation, the constric- 

 tions being only lost as they disappear gradually at the root of 

 the stolon. 



