214 ' GEORGE W. TANNREUTHER 
it separates the primary and secondary mesoderm. The ex- 
treme ends of the digestive tract are ectodermal. Among other 
annelids and also in molluscs, so far as is known, the entodermal 
cells are not broken up into cells but enter directly into the for- 
mation of the digestive tract. 
The posterior cell, d? (X), of the second generation of ecto- 
meres is the largest cell of the segmenting ovum. The deriva- 
tives of X are symmetrically placed with reference to the median 
plane of the future individual. The large cell, X, gives rise to 
the trunk ectoderm, the nervous and the excretory systems. 
The nervous system is derived from the two neuroblasts. The 
brain is formed from the extreme anterior end of the neural rows. 
The largest cell, d* (M), of the fourth generation of micro- 
meres, gives rise to the entire mesoderm. It is the first cell to 
divide in a bilaterally symmetrical manner. The primary 
mesoblast cells, M, M, bud off five or six small cells each, beneath 
the first quartette of ectomeres, which give rise to the secondary 
mesoderm on the dorsal side of the embryo. Immediately 
after these small cells are budded off, the primary mesoblasts, 
by a teloblastic proliferation of cells, produce the mesoblast 
bands. 
The embryo increases but little in bulk before the germ bands 
are formed. The embryo as a whole, during its early stages of 
development, is extremely plastic and may vary considerably 
in its transverse and longitudinal axes. The developing embryo 
is completely turned on itself, and the anterior and posterior 
ends are in immediate contact. The outer surface is ventral 
and the turned in portion is dorsal. This peculiarity of develop- 
ment is foreshadowed in the position taken by the early cleavage 
cells. 
At the beginning of the germ-band formation, the embryo 
begins to rotate on its transverse axis. This movement is due 
to the action of cilia, which are produced by the ectodermal 
cells on the median ventro-anterior end of the embryo; the ro- 
tation alternates. 
As growth continues within the cocoon, the ends of the em- 
bryo soon begin to overlap. The embryo may assume almost 
