174 GEORGE W. TANNREUTHER 
Figs. 10-13 Diagrammatic figures to show the ventro-posterior extension of 
the ectomeres, in the closure of the blastopore. 
Fig. 10 Thirty-three cell stage, taken a little to the right of the median 
plane. 
Fig. 11 Vertical section to the left of the median plane. 
Fig. 12 Vertical section of a ninety-cell stage. 
Fig. 138 Vertical section of an embryo at the time of closing of the blasto- 
pore. The mesoblast bands have just begun. 
The heavy stippling represent endoderm; the light stippling mesoderm, and 
the unstippled the ectoderm or ectomeres. m, secondary mesoblast; M@, meso- 
blasts; bl, blastopore; ble, point where the blastopore closes; X, derivatives of 
the ‘first somatoblast;’ mes, mesoblast bands. 
meso-teloblasts are represented by an undifferentiated mass of 
cells, which later give rise to the musculature of the last three 
segments of the worm, and are directly concerned in the forma- 
tion and movements of the posterior sucker. 
