260 T. H. Burtexp, 
Fig. 15 (Section 2,4). On the right side the sect. passes through 
the anterior region of proto. IX (left side): on the left side through the 
posterior region of proto. VIII (right side). 
Fig. 16 (Section 2.5). Shows the rudiment in an intersegmental 
region on the left side of the figure. 
Fig. 17 (Section 2,6). On the right of the figure the pronephros 
in the middle of somite IX (left side) is visible. 
Fig. 18 (Section 2,7). On the right side of this section the last 
trace of the pronephros on the animal’s left side is seen. 
Figs. 19—41 are from successive sections of embryo D. 
Fig. 19 (Section 2,16). The anterior limit of the pronephrie rudiment 
on the animals right side is observable in this section, which passes 
through the front end of protovertebra VII. 
Fig. 20 (Section 2,17). On the right side of this figure is seen 
the intersegmental region between nephrotomes of the sixth and seventh 
somites. 
Fig. 21 (Section 2,18). In this figure (and in all the others of this 
series) the splanchnic mesoderm of the lateral plate of the left side is 
further away from the endoderm than is the case on the right side. At 
a later stage the large vitelline artery will be formed in this region. 
Fig. 22 (Section 2,19). The anterior limit of the left pronephric 
rudiment is visible on the right side in this section. 
Fig. 23 (Section 2,20). Passes through the region of somite VII 
on each side. 
Fig. 24 (Section 2,21). The real nature of the pronephric rudiment 
is disguised in this and the succeeding figure, but its origin is obviously 
from the lateral plate mesoderm. 
Plate 20. 
Fig. 25 (Section 2,22). This and the next section cut obliquely 
through the lower posterior end of the seventh somite on the embryo’s 
right side. 
Fig. 26 (Section 2,23). The groove-like nature of the early pro- 
nephros is still visible on the right side of the figure. 
Fig. 27 (Section 2,24). The pronephros on the left side of the 
figure is very prominent, notwithstanding the fact that it is situated 
between somites VII and VIII. 
Fig. 28 (Section 2,25). On the right of this figure it is not difficult 
to observe the relations of protovertebra, nephrotome and lateral plate 
regions, the pronephros being situated in the last-mentioned. 
Fig. 29 (Section 2,26). Notice the close proximity of the distal end 
