The openings of the nasal pits are at first single; but the opening of each becomes gradually divided into two by the growth of a flap on the outer side (fig. 54, _ol_). It is probable that this flap is equivalent to the fold of the superior maxillary process of the Amniota, which by its growth roofs over the open groove which originally leads from the external to the internal nares; so that the two openings of each nasal sack, so established in these and in other fishes, correspond to the external and internal nares of higher Vertebrata. [FIG. 55. VENTRAL VIEW OF A LARVA OF ACIPENSER OF 11 MILLIMETRES. _m._ mouth; _st._ suctorial (?) processes; _op._ eye; _g._ gills.] At the time of hatching there is a continuous dorso-ventral fin, which, by atrophy in some parts, and hypertrophy in other parts, gives rise to all the unpaired fins of the adult, except the first dorsal and the abdominal. The caudal part of the fin is at first symmetrical, and the heterocercal tail is produced by the special growth of the ventral part of the fin. [FIG. 56. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE ANTERIOR PART OF THE TRUNK OF A LARVA OF ACIPENSER TO SHEW THE POSITION OCCUPIED BY THE YOLK. _in._ intestine; _st._ stomach filled with yolk; _oes._ oesophagus; _l._ liver; _ht._ heart; _ch._ notochord; _sp.c._ spinal cord.] Of the internal features of development in the Sturgeon the most important concern the relation of the yolk to the alimentary tract. In most Vertebrata the yolk-cells form a protuberance of the part of the alimentary canal, immediately behind the duodenum. The yolk may either, as in the lamprey or frog, form a simple thickening of the alimentary wall in this region, or it may constitute a well-developed yolk-sack as in Elasmobranchii and the Amniota. In either case the liver is placed in front of the yolk. In the Sturgeon on the contrary the yolk is placed almost entirely in front of the liver, and the Sturgeon appears to be also peculiar in that the yolk, instead of constituting an appendage of the alimentary tract, is completely enclosed in a dilated portion of the tract which becomes the stomach (figs. 56 and 57). It dilates this portion to such extent that it might be supposed to form a true external yolk-sack. In the stages before hatching the glandular hypoblast, which was established on the dorsal side of the primitive mesenteron, envelops the yolk-cells, which fuse together into a yolk-mass, and lose all trace of their original cellular structure. The peculiar flattening out of the embryo over the yolk (_vide_ p. 105) is no doubt connected with the mode in which the yolk becomes enveloped by the hypoblast. [FIG. 57. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A LARVA OF ACIPENSER 5 MM. IN LENGTH.