176 GEORGE W. TANNREUTHER 
these rows of cells form a part of the general ectoderm, being 
partly covered here and there by adjoining cells. In later stages 
of development, these rows of cells become completely covered 
as they gradually sink beneath the surface, and thus come to 
lie between the mesoblast and the ectoderm or ectoblast. 
Fig. 17 Twelfth section from the top, to show the anterior extension of the 
mesoblast bands below and around the entoderm. 
Fig. 18 Nineteenth section from the top, to show the lower side of the meso- 
blast. The section passes below the entoderm. The spindles represent the 
beginning of the first division of the neuroblasts to form the neural rows. 
Heavy stippling represents entoderm; light stippling mesoderm and the 
unstippled portion the ectoderm. The sections of figures 15-18 were eight micra 
thick. There were 21 sections in all. a, anterior, and p, posterior, represent the 
respective ends of the cleavage cells, but not the future ends of the embryo; 
r, right; 1, left; ent, entoderm and ect, ecoderm. 
3. OUTER STRATUM OF THE GERM BANDS 
This stratum forms the definitive ectoderm and needs no 
further description at this point of development. 
The embryo now elgongates very rapidly, and the general 
shape of the adult worm becomes recognizable. The telo- 
blasts become less and less distinct, until finally the cell rows 
terminate posteriorly in a group of small cells. The meso- 
teloblasts extend farther posteriorly than the neuro-teloblasts. 
New cells are always formed from the anterior surface of the 
teloblasts. There can be no question as to the origin of the 
germ bands from corresponding teloblasts, as their formation 
