236 T. H. Burtenp, 
Although embryo F has about 12 protovertebrae more than has 
embryo E the changes in the condition of the pronephros or its 
duct are not extensive during this interval in the development. 
Thus a comparison of embryos E and F shows that the pronephric 
duct has grown back over one segment, and the embryo has become 
longer only by '/; mm. 
The first pronephric artery on the right side is found in sections 
2,34—2,37 (viz. between segs. VI and VII): the second artery is 
present in sections 2,42—2,46 (inclusive), viz. between segs. VII 
and VIII, whilst the third artery is a small one in sections 2,53 
and 2,54 (viz. at the posterior end of somite VIII. On the left side 
there are indications of blood-vessels intersegmentally in the region 
between VI and VII, and at the front and hind ends of somite VII. 
Owing to the fact that the dorsal portion of each protovertebra 
tends to slope backwards, and also since the nephrotome region 
assumes an oblique position though originally vertical, it is necessary 
to take the front median limit of each somite in the nephrotome 
region as the original front end of each somite. 
By a careful comparison of Figs. 19 to 73, it will be seen that 
section 2,39 (Fig. 51) of this series corresponds with section 2,20 
(Fig. 23) of Embryo D. See Plates 21 to 24, Figs. 43 to 73. 
Embryo G. 
6 mm long. Stained in Boraxcarmine. Mounted upon 4 slides 
having respectively 97, 102, 116 and 150 sections. 45 protoverte- 
brae. 
Left side Right side 
1,64— 1,854 Ear 1,65 —1,84 
2,15—2,29 I 2,18— 2,28 
2,30— 2,39 IT 2,29—2,40 
2,40— 2,50 III 2,36— 2,51 
2,51—2,60 IV 2,49—2,62 
2,61 —2,68 V 2,60— 2,72 
2,69—2,77 VI 2,11—2,80 
2,18—2,87 VII 2,79—2,89 
2,86—2,96 VII 2,88—2,96 
2,94 — 3,2 IX 2,94—3,3 
3,2—3,10 X 3,2—3,11 
Segmentalbläschen in segs. II? III to XVI or XVIL 
On the left side the pronephros begins at 2,81; the duct becomes 
