DEVELOPMENT OF BALANOGLOSSUS KOWALEVSEKIT. 121 
slits.) (The section is not truly vertical, as*it cuts the gill-slit.) It exhibits 
the relation of the notochord and other parts. (Obj. CC, oc. 2.) 
Fic. 46.—By an oversight no figure bearing this number appears in the 
plates. The figure referred to as such in the text at pages 85, 98, and 103, 
is Fig. 45. 
Fic. 47.—A transverse section of the extreme tip of the notochord, &c., 
in the larva from which Figs. 31—45 were taken. It shows the histology of 
the notochord and the arrangement upon it of the mesoblastic tissues, which 
are pushed in by it. (Obj. D, oc. 2.) 
Fic. 48.—Transverse section across the proboscis pore of a larva with three 
gill-slits. The two skeletal rods are here fused and have attained a considerable 
size. (Obj. F, oc. 2.) 
Figs. 49—53 represent transverse sections taken from B. Kowalevskii, 
at the stage of four pairs of gill-slits. (When the remaining structures are 
dealt with a fuller explanation will be given of these and of the subsequent 
figures ; on the present occasion they are only introduced to explain the account 
of the notochord given in the text.) Figs. 49—53 are numbered from before 
backwards. 
Fies. 49 and 50 illustrate the histology of the anterior end of the notochord. 
(Obj. F, oc. 2.) These sections are in front of the heart. 
Fic. 51 shows a section of the whole proboscis, to illustrate the relations of 
the parts. The body cavity may be seen to be nearly filled up with muscle- 
fibres and connective tissue, with the exception of a small central space, in 
which lies the notochord bearing the central mesoblastic structures, viz. the 
heart and proboscis gland with its sac. (Obj. C, oc. 2.) 
Fic. 52 exhibits the central structures of Fig. 51. (Obj. F, oe. 2.) 
Fie. 53.—Here the lumen of the notochord is reached. At this point the 
sac of the proboscis gland is attached dorsally to the skin of the proboscis. 
(Obj. F, oc. 2.) 
Fig. 54.—Transverse section of proboscis stalk of an older animal, taken 
behind the epiblast sac of the proboscis pore, the back of which appears in 
section. The skeletal rod has still in this region two central “cores.” (Obj. 
D, oc. 2.) 
Fic. 55.—A transverse section of the notochord of an adult B. Kowa- 
levskii, taken in front of its lumen (ep. Fig. 57). (Obj. CC, oe. 2.) 
Fie. 56.—Transverse section of the same as foregoing, through the dilated 
lumen in front of the skeletal rod. 
Fic. 57.—Longitudinal vertical median section of the back of the proboscis 
and the front of the collar of B. Kowalevskii, to show the relation of the 
notochord. The other parts are diagrammatic. (Obj. A, oc. 2.) 
Fic. 58.—Transverse section of the junction of the proboscis stalk with 
the collar. (Semi-diagrammatic.) Exhibits relations of the notochord and its 
sheath to the skeletal rods, nervous system, &c. (Obj. A, oc. 2.) 
