a2 GEORGE W. TANNREUTHER 
As the embryo elongates, the entodermal cells increase in number. 
This process of growth is continued until the digestive tract is 
completely formed. Figure 99 shows the anterior and posterior 
limits of the digestive tract, which is formed from the four en- 
tomeres. The anterior end shows a distinct lumen, while the 
posterior end is yet a solid mass of cells. The entire digestive 
tract, except the insignificant stomodaeum and proctodaeum, 
is entodermal in origin. The proctodaeum is not formed until 
the time of hatching. It occurs on the dorsal side of the tenth 
segment. The embryo is completely turned on itself (fig. 99) 
and brings the anterior and posterior ends of the digestive tract 
in close proximity. The differentiation of the digestive tract 
begins anteriorly and progresses posteriorly. As growth con- 
tinues, the outer cells of the entodermal mass form an epithelial 
layer. At first the cells are somewhat flattened, but soon take 
a columnar position, and form the columnar epithelium of the 
digestive tract. The cells within soon lose their staining proper- 
ties, break up and serve as food for the developing embryo. 
The digestive tract in its course of development, passes through 
the following stages: the first stage is represented by the four 
entomeres A, B, C and D; the second stage by a solid mass of 
cleavage cells (the cell boundaries are often very indistinct) 
within the center of the embryo; the third stage by an elonga- 
tion of the entodermal mass as the larva lengthens, and by the 
establishment of a lumen; the fourth stage by a thin layer of 
flattened epithelium and later a columnar epithelium; the fifth 
stage, the cells within the epithelial layer serve as food; and sixth 
the formation of the stomodaeum and proctodaeum. 
GENERAL HISTORY OF THE GERM BANDS 
The term ‘germ bands’ has been variously interpreted by differ- 
ent investigators on cell lineage. The term germ bands, or the 
German. equivalent ‘Keim Steifen,’ is usually restricted to the 
strata derived from the teloblasts, the ectoblastic layer being 
excluded. It is held by others that the germ bands of annelids 
are purely mesoblastic. 
