DESCRIPTION OF PLATES 
All drawings were made with a camera lucida under a magnification of about 
125 diameters. All whole amount drawings, with one or two exceptions, were 
made from the living egg. The variation in size of the surface views is due to 
a difference in the size of the eggs. The sections were not uniformly magnified. 
Stippling has been adopted for the sake of clearness. 
REFERENCE LETTERS 
a., anterior mi.a., Minor annulus 
bl., blastopore mj.a., Major annulus 
c., ciliated cells mo., mouth 
c.c., cleavage cavity p., posterior 
c.l., cerebral lobes pb', polar bodies 
coe., coelom ph., pharynx 
€.cp., egg capsule pr., proctodaeum 
ec., ectoderm so.mes., somatic mesoderm 
en., entoderm sp.mes., splanchnic mesoderm 
gn., ganglia st., stomodaeum 
mes., mesoderm v., ventral 
A, left macromere 
B, anterior macromere 
C, right macromere 
D, posterior macromere 
a!, b!, ce, d!, at!, ete., first generation of ectomeres 
a”, b?, c?, d?, a2-1-1, ete., second generation of ectomeres 
a3, b3, c3, d3, a3-1, etc., third generation of ectomeres 
at, b4, c*, d+, ete., fourth generation of micromeres 
X = d? first somatoblast 
X, X, right and left proteloblasts 
X@, neuroblast 
X(2), X(3), X4), nephroblasts 
M = d* second somatoblast 
m, secondary mesoblast 
x!_x’, small derivatives from X 
N, posterior end of nephric rows 
Ne, posterior end of neural rows 
nc, neural rows 
np, nephric rows 
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