234 T. H. BurLEN», 
mesenterica of some writers) which is prominent in later stages of 
the development on the right side, and which seems to have an 
important influence upon the later stages of development of the 
right pronephros. 
Embryo E. 
5 mm long. 28 or 29 protovertebrae. Mounted upon 3 slides having 
respectively 131, 147 and 141 sections. 
Segmentalbläschen in segs. II?, TII—XII or XIV. 
Left side Right side 
1,72—1,93 Ear 1,75—1,93 
—1,122 I 1,109— 1,124 
1,122—1,128 II 1,124—2,1 
1,128—2,10 LE 2,1 —2,12 
2,10 —2,23 IV 2,12—2,25 
2,23—2,33 V 2,25—2,34 
2,34—2,43 VI 2,34—2,44 
2,44— 2,52 VII 2,44 — 2,52 
2,53—2,61 VIII 2,52—2,61 
2,62—2,70 IX 2,61—-2,71 
2,71—2,79 x 2,71—2,81 
Neurenteric canal 3,127—3,134. 
The kidney rudiment, in the form of a groove in places and a 
solid outgrowth in others, is present on the left side from 2,44 to 
2,67. The duct begins at 2,68 and ends at 2,115 (viz. end of seg- 
ment XV). 
On the right side the rudiment is present from 2,44 to about 2,72. 
The duct is separated from the underlying mesoderm from 2,67 to 
2,105 (viz. end of segment XIV): 
The pronephros is confined in this embryo to segments VII, 
VIII and IX. Section 2,44 of this series (Fig. 42) corresponds closely 
with section 2,17 (Fig. 20) of embryo D, and seems to indicate the 
same region of the body. The pronephros appears therefore to 
become reduced in extent at a very early stage, and to end post- 
eriorly in segment IX. 
The only blood-vessel distinguishable is found in sections 2,51 
and 2,52 (viz. between segs. VII and VIII) on the left and right 
sides. There may also be a branch from the aorta in 2,59 and 2,60. 
