LANCELET-FISHES. 
1217 
(gl) on the right side of the intestinal canal becomes 
a gland * * 6 which also disappears at the close of the larval 
period, and whose outer efferent duct bends round be- 
low to the left side of the body (fig. 375). But in the 
median ventral line the intestinal canal coalesces at one 
point after another, backwards from in front, with the 
wall of the body, becomes thickened there (fig. 374, at I), 
and eventually opens into gill-slits (fig. 375), which are 
first moved up to the right, but afterwards migrate to 
the left side of the body. In the meantime, however, 
new gill-slits (fig. 378) have been similarly formed in 
Fig. 374. Optical section of a larva with its protovertebrse, seen from the right. After Hatschek. X 280. 
Here the lateral dilatations of the foremost part of the intestinal canal have diverged widely; the right ( dvd ) is elongate and thin-walled, 
the left (dvs, which from that side shines through in the figure, at about the middle of the former) has thicker walls (deeper cells) and is 
round. Both separate by constriction from the intestinal canal, the anterior end of which is thereby thrust further back. A glandular growth 
(the so-called club-shaped gland of Hatschek, gl) has besides begun to develop on the inside of the intestinal canal by means of a trans- 
versal constriction, first canaliculate and afterwards, owing to the coalescence of the edges of the groove, tubiform, of the right side of the in- 
testinal wall behind the said lateral dilatation. Behind this gland the wall of the intestinal canal has been thickened in the inferior (ventral) 
median line of the larva and has coalesced with the ectoderm at a. point (I) where the first gill-opening afterwards appears. As yet there 
is no true mouth; and the intestinal canal is continuous behind with the cerebrospinal. 
ch oc cil m 
Fig. 375. A larva l 1 / 2 mm. long, seen from the left. After Ray Lankestek and Willey. 
Of the four gill-slits, which are situated on the right side (I — IV), the foremost three are rather large, the fourth is rudimentary. The 
first is visible through the posterior part of the semioval mouth aperture, which belongs to the left side of the body. Through the anterior 
part of this aperture are visible, also from the right side of the body, both the club-shaped gland (gl), the outer orifice of which may be 
seen below, on the wall of the body below the mouth aperture, and, just in front of the said gland, a transversal, but curved thickening (end), 
bisected along the middle by a groove, the future endostyle. Before the upper edge of the mouth aperture appears the opening of the funnel- 
shaped, ciliated organ (cil). Above this and from the anterior to the posterior end of the body (in front of the caudal fin) lies the notochord 
(ch), divided into its numerous transverse disks. Above the notochord, and posteriorly round the termination thereof, the spinal cord (m) is 
extended, with the large pigment spot (ocular rudiment, oc) at its anterior end and some smaller sparse pigment spots on its sides. In the 
lanceolate caudal fin a sparse collection of pigment spots is also visible. The number of the myomeres is 36; the first myocomma runs 
obliquely across the anterior edge of the ocular rudiment and down along the subjacent part of the notochord. In the posterior half of the 
intestinal canal (below the 15th and 16th myomeres) the wall of this canal is thickened (v), an indication of the rudimentary stomach. 
The anal aperture (an) lies on the left side of the body. 
lopment as in the higher vertebrates". Between the 
first two germinal layers an intermediate layer (the 
mesoderm. , fig. 372, A') is formed; and now that the 
intestinal canal and an incipient abdominal cavity have 
begun to differentiate themselves, there develop a noto- 
chord and, by means of bulgings and constrictions of 
the intestinal canal, primitive vertebrae ( protovertebrce ), 
which increase their number backwards from in front. 
The foremost bulges (figs. 373 and 374) meet with a 
singular fate. The right (dvd) dwindles into the hol- 
low at the tip of the snout. The left, on the other 
hand, develops into a funnel-shaped, ciliated organ, 
probably an organ of smell, which disappears, however, 
at the termination of the larval period. A special bulge 
a row higher up on the right side, and these become 
the permanent gill-slits on that side. This only ap- 
plies, however, to the anterior part of the branchial 
a Branchiostoma differs, however, from the higher vertebrates in that the rudiment of the spinal cord is separated from the circum- 
jacent cells of the ectoderm before the canal is closed above (Hatschek). 
6 See Willey, Quart. Journ. Micr. Sc., n. ser., vol. XXXII, pt 2 (No. CXXVI, March 1891), p. 209. 
