A22 LEFEVRE. 
EXPLANATION OF PLATE XXXI. 
In all the figures of this plate the ectoderm is represented by a line, and the 
elements of the blood have been omitted for the sake of simplicity. 
Fics. 16-21. Series of transverse sections of a bud at about the same stage 
as that shown in Fig. 4, Pl. XXIX. The sections are taken respectively at the 
levels indicated by the lines a, 4, c, d, e, and f, in Fig. 4. Fig. 16, line a, the most 
anterior section, shows undivided portion of branchial sac, 4y.s., and dorsal tube, 
dt. Fig. 17, line 4, is a little further back, and shows anterior extension of right 
peribranchial sac, v.a.ex. Fig. 18, line c, is taken immediately in front of the cloacal 
cavity, and shows the extensions of the peribranchial cavity on both sides. The sec- 
tion passes through the extreme posterior end of the dorsal tube,d.¢. Fig. 19, line 
d, passes through the anterior end of the cloacal cavity, c/., which is seen to con- 
nect the lateral portions of the peribranchial cavity. Fig. 20, line e, is taken from 
the posterior end of the bud, and shows the connection of the left peribranchial 
sac with the stolonic partition at stc. The pericardial sac, Zc., is also shown in 
this region. Fig. 21, line 7, shows the undivided extreme posterior end of the 
branchial sac. The section passes through a portion of the intestine, zzz¢., and 
behind the connection with the stolonic partition. X 200. 
FIGs. 22-24. Series of transverse sections of a bud of about the same age as 
the one represented in Fig. 5, Pl. XXIX. Fig. 22 is a section through the anterior 
region, and shows the extensions of the peribranchial sacs, ~.a.ex. and /.a.ex. 
Fig. 23 is drawn from a section which is taken from the middle region of the bud, 
and shows the median portion or cloaca, c/., connecting the lateral divisions of the 
peribranchial cavity, 7.4.5. and /.44.5., which is now entirely cut off from the 
branchial sac, 4ér.s. Fig. 24 shows the posterior extensions (epicardial sacs ?) of 
the peribranchial cavity, ~.p.ex. and /.f.ex.,and the remnant of the connection 
with the stolonic partition, 7.st.c. X 200. 
