344 
K. \V. MACT.KTDK. 
tlie closed front end of the trunk-cavity. The collar-cavity is still open 
to the gut on the riglit side ; in fig. 12 c it is closed off on both sides. In 
fig. 12 d the posterior ends of the trunk-cavities are seen to open into 
the ai’chenteron. Magnification 900 diameters. 
Fig. 13. — Transverse section through the front end of a larva in 
which four somites have been cut off from the front end of the trunk- 
cavity to show the origin of the head-cavities, pore. The neuropore. 
l.li. Left head-cavity, r.li. Right head-cavity. Magnification 900 
diameters. 
Fig. 14. — Transverse section through the front end of a larva in 
which five somites have been cut off from the front end of the trunk - 
cavities. The head-cavities are more advanced in development, but still 
open into the gut. r.coll., l.coU. The anterior ends of the right and 
left collar-cavities respectively. Magnification 900 diameters. 
Fig. 15. — Transverse section through the front end of a larva about 
two days old to show the persistent connection of the left collai’-cavity 
with the pharynx, out of which it appears probal)le that Hatschek's 
neplu-idium {nepli.) developed. Magnification 1000 diameters. 
Figs. 10 n-f/. — Seven transverse sections through a larva of about the 
same age as that represented in fig. 15 before the mouth is open. In 
fig. 16 a the still open neuropore (pore) is seen, and below the notochord 
the thin-walled right head-cavity. In fig. 10 h the left head-cavity is seen 
completely closed off from the gut. and below it the right head-cavity. 
At V. a vacuole is seen in an ectoderm cell, which marks the spot 
where this cavity will eventually acquire an opening to the exterior. 
In fig. 16 c the thickening of the ectoderm (m.) is seen, which marks the 
spot wheie the mouth will break through. The collar-cavities are seen 
at the sides of the gut extending towards the mid-ventral line. In fig. 10 d 
the club-shaped gland is seen originating as a hollow outgrowth from 
the wall of the pharynx, and the posterior extensions of the collar- 
cavities are seen below the splaucbnoceles, which have been foianed by 
the fusion of the ventral jDarts of the posterior somites, my. The 
myocele of one of the myotomes. muse. Muscular fibres on the inner 
walls of these myotomes. In fig. 16 e the first gill-slit is seen to originate 
by the meeting of an ectodermal in-growth (br.ect.) and an endodermal 
outgrowth (br.end.). In fig. 16/ the anal diverticulum is seen, and the 
expansion of the ectodermal cells to form the caudal fin (/a), which is 
seen as a cuticular rim. In fig. 16 gr the solid seam of cells representing 
the neurenteric canal is seen. Magnification lOOO diameters. 
PLATE 21. 
Figs. 17 a-h . — Eight transverse sections through a larva in which the 
month and first gill-slit have been formed, and in which the left head- 
