260 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
be developed, and is very conspicuous as a delicate but low fold on the top of the yolk- ' 
sack, a little distance from the side of the body at /, Fig. 18. 
In Fig. 18 there is no outwardly visible evidence of branchial clefts as in Figs. 
12 and 13 of the sterlet, but the opercular fold op already conceals the visceral arches 
from the outside. The tail fold is also much wider in the recently hatched embryo of ■ 
the common sturgeon, as Fig. 18 clearly shows. A single spacious Uuvierian duct, 
c V, carries the blood from the head and body down over either side of the yolk to 
empty it into the inferior or venous end of the heart, H, which lies in a spacious con- 
cavo-convex cavity p e, within the anterior end of the somatic wall of the yolk-sack. i 
This space is continuous with that of the general body-cavity posteriorly. 
No barbels are developed at this stage in the young of the common sturgeon, , 
nor are they present in the same stage of the young sterlet. The nasal sack is now 
a simple depression in the epidermis of both. The various portions of the brain are 
now differentiated, while the notochord is formed and the lateral muscle plates are 
numerous, nearly twice as many in fact as in most young Teleosts of the same stage. 
In this respect the sturgeons resemble the Elasmobrauchs rather than the group of 
Ganoids or Teleosts. Only a few of the long-bodied Teleosts have the muscle plates 
very numerous in the embryo. 
The spiracular cleft is very obvious in the recently hatched sturgeon, as indicated ' 
in Fig. 18 at sp. The eye is relatively very small, a feature in which the embryo of 
the sturgeon agrees with the embryonic lampreys and batrachia more closely than ; 
with any fishes except Lepidosteus. The mouth appears very far back from the tip of 
the snout and is at first almost completely concealed from below as a narrow, traus- I 
' I 
verse cleft in the angle formed by the upi^er anterior extremity of the yolk-bag and j 
the under side of the head of the recently hatched larva. I 
Later, as the head lengthens and the yolk-sack is absorbed to some extent, the 
mouth becomes obvious from beneath as a wide transverse opening with a row of i 
about ten formidable teeth in each jaw, as shown in Figs. 14, 15, and 17. The barbels 
now appear as four papill® placed in a transverse row in front of the mouth, as shown 
in Figs. 15 and 16. In trout of the row of barbels there is a depression, seen in Fig. 
15, which may be the homologue of the preoral disk found in Lepidosteus and Amia 
by Mr. A. Agassiz and Mr. Allis.' 
The branchial filaments now begin to grow rapidly so as to be extended beyond 
the posterior margins of the gill covers. They consequently become visible from the 
side, as shown in Figs. 14, 15, and 16. The yolk is now almost absorbed and the 
ventral pair of fins begins to be evident, as shown in Fig. 16, while the pectoral is 
quite large, but still rounded at the margin and not pointed as in the adult. A notch ■ ' 
between the dorsal and caudal begins to separate these two, while the same thing is 
occurring on the ventral side, so as to separate the lower lobe of the caudal from the 
anal. The partial atrophy of the median fin fold in front of the rudiment of the dorsal 
now begins, as shown in Fig. 16. At the same time the extensive median preanal fin- 
fold begins to be absorbed. 
In the head region the barbels are becoming more conspicuous, while the snout is 
longer but still remains rounded off. The nostrils are beginning to become divided 
externally by the upgrowth on the dorsal and ventral borders of the nasal sack of two 
processes which will eventually fuse and form a bridge running diagonally across it 
as in the adult. 
