118 WILLIAM BATESON. 
the pharynx. Neh. Notochord. JV. pr. Neural pore. Op. Operculum. 
pkt. Fibrous substance of the nervous system. P. pr. Proboscis pore. 
P.h.b.c. Perihemal body cavity. P. sk. Skin of proboscis. Sep. Septum 
between the horns of the anterior body cavity. SA. Sheath of notochord. 
Skr. Sucker. Sé. Skin. S.7. Supporting rod. 8%. Skin. Sp. Tissue-space 
in the proboscis cavity. és. Testis. V.vs. Ventral vessel. V. 2. s. Ven- 
tral nervous system. Viv. Valve of gill-slit. «. Pyriform cells of splanch- 
nopleure of middle body cavity. 
With the exception of Figs. 5 and 6, the outlines were all drawn with 
Zeiss’s camera lucida. I have to thank Mr. Edwin Wilson of the Lithographic 
Department of the Cambridge Scientific Instrument Company for drawing for 
me two beautiful figures (Figs. 5 and 6) of the whole animal from preserved 
specimens, and also for Figs. 1, 2,3, and 4, which he has prepared for me 
from my own sketches, the original outlines of which were traced from living 
specimens. 
Fic. 1.—Whole animal seen from the side, immediately after the appearance 
of the second pair of gill-slits. (Obj. A, oc. 2.) 
Fre. 2.—Similar view of an older animal, with two gill-slits. (Obj. A, oc. 2.) 
Fie. 3.—Similar view of a three-gill-slit larva. (Obj. A, oc. 2.) 
Fic. 3a.—Posterior end of the same in a retracted state, seen in profile 
(from a preserved specimen). (Obj. A, oc. 2.) 
Fic. 4.—Side view of larva with five pairs of gill-slits. The fold of the 
operculum covering part of the first gill-slit is semi-transparent. (Obj. A, oc. 2.) 
Fic. 5.—Side view of preserved specimen with nine pairs of gill-slits. 
Owing to the contraction of the body and the protrusion of some of the 
valves, few only of these are visible. (Obj. AA, oc. 2.) 
Fic. 6.—The adult animal (¢). (x 2 diameters.) 
Fig. 7.—Longitudinal vertical section (not quite median) of a larva in 
Stage F. (Obj. C, oc. 2.) 
Fie. 8.—Cells of mesoblast of anterior pouch, from a transverse section of 
a larva shortly after this pouch is closed off from the hypoblast. (Obj. F, 
oc. 2.) 
Fic. 9.—The same tissue from the posterior third of the proboscis of a 
rather older larva, showing the proliferation and commencing differentiation of 
the mesoblastic elements. (Obj. D, oc. 2.) 
Fic. 10.—Section similar to Fig. 7, from a rather older larva to show 
anterior, dorsal structures (Stage G—H). (Obj. D, oe. 2.) 
Figs. 11—20 represent transverse sections of a larva in Stage G—H, with 
the exception of Figs. 12 and 16, which were drawn under Obj. F, oc. 2. All 
these figures were drawn under Obj. CC, oc. 2, In most of these figures the 
ectoderm is only indicated ona short arc of the circle of the whole section, 
They are numbered from before backwards, 
