184 GEORGE W. TANNREUTHER 
very rapidly and gradually become covered by the ectodermal 
cells as they sink beneath the surface (figs. 66-72), and form the 
middle stratum of the germ bands. In a nearly median longi- 
tudinal section (fig. 99), the relation of the parts are shown. 
The neural plate at different points shows the formation of 
ganglia. The anterior end of the section passes through the 
exact median plane and does not show any ganglia. The neuro- 
blasts still persist at the posterior end. 
The entire nervous system arises as two simple longitudinal 
rows of cells, and each row is produced by the continued pro- 
liferation of cells from a single cell, ‘the neuroblast.’ This proof 
is established by the study of surface preparations in connection 
with sections taken in different planes through the developing 
neural elements. The neural rows can be followed to the ex- 
treme anterior end, where they pass up around the pharynx 
and. give rise to the cerebral ganglia on either side (fig. 96) by 
a thickening of the anterior extremity of the corresponding 
neural rows. There are exactly four rows of cells in the middle 
stratum of each germ band. The outline of the rows can be 
easily seen in surface views of the living embryo (figs. 63-66). 
They are more distinctly marked at the posterior ends, and 
become less distinct anteriorly, which is due to the more ad- 
vanced development; that is, each row becomes double, then 
triple, ete. (figs. 68-69) and at the same time, its boundary 
lines become less distinct. . 
2. THE EXCRETORY SYSTEM 
After the bilateral division of the ‘first somatoblast,’ each 
proteloblast contains the neural and nephric elements of their 
respective sides. According to Whitman, these two cells are 
called the neuro-nephroblasts. But when each _proteloblast 
X, X divides equally the neural and nephric elements become 
separated, X neural and X nephridial (figs. 47-48). The 
nephroblast X on either side next buds off a very small cell x’, 
which becomes ectodermal (fig. 49). Immediately after the for- 
mation of this small cell, X‘? divides nearly equally, and forms 
