156 
MR. W. K. PARKER OK THE STRUCTURE AND 
the brain is above and in front of the distinct pituitary body (fig. 1, py.), and seems to 
be distinct from the fore-brain (C 1 .); this, however, is due to the manner in which it 
has been cut through on its projecting part. In this section the rudimentary hemi¬ 
spheres (CP) are cut through where they pass forward from the first cerebral vesicle 
(figs. 1 and 2, C 1 ., CP). The small olfactory sacs (ol.) are cut across; they lie outside 
the junction of the hemispheres with the thalamencephalon ; the eye-balls (e.) are cut 
through in the lower part. 
Section 3.—The lowest section (Plate 14, fig. 3) is through the lower part of the 
curved head, and takes in the whole boundary of the mouth (m. ; see also fig. l). 
The olfactory capsules are removed, and the barbels (bb.) are cut across at their root; 
each, at present, contains a pith of true cartilage. The fore part of the pterygo¬ 
palatine and the quadrate region of the same (pg.q., q-) are shown as severed, and the 
latter is seen as a transverse process, bulging below to form the hinge of the jaw. 
Behind the mouth (m.) the lower lip is seen with its teeth (l.l., t.), and in the substance 
of this visceral fold the whole length of the mandibles (mh) is shown, first thick, in 
the articular region, then slenderer, and again enlarging distally. 
Third Stage (continued).— Transversly vertical sectio7is of the head of the largest larva 
of Acipenser ruthenus ; 1 millims. long. 
Section 1.—The first section (Plate 14, fig. 7) is through the barbels (bb.) at right 
angles to the horizontal section just described (fig. 3), and also to the axis of the down- 
bent brain which is here developed into the rudimentary hemispheres (CP). The 
plane of this section would form an angle of 45° with the plane of a section taken 
vertically through the hind part of the hind-brain, or through the front of the yolk 
mass and heart in such a larva as is shown in a longitudinally-vertical section (Plate 14, 
fig. 1, C 3 ., y., h.). That, however, was a younger larva, and the head in this had 
straightened a little. 
The hemispheres (fig. 7, CP) are here shown to be, together, sub-oval in section, 
somewhat grooved above, and slightly protuberant below. The olfactory sacs (ol.) have 
no roof in front, and their floor is formed by the ethmo-palatine cartilage (e.pa.), which 
is distinct, in front, from the trabecula. The skin of the fore face, however, is developed 
into a valvular fold around the opening, and the mucous membrane is applied to the 
rising and falling of the structures that encompass it; there is no proper “ paraneural ” 
cartilage over the nasal capsule; in this, Acipenser agrees with Lepidosteus and the 
Teleostei, and differs from the Selachians, Cyclostomes, and Amphibia. Here the floor 
(e.pa.) is flat and distinct, and there is a considerable space between it and the 
trabecula (tr.) which is thick at its outer edge, and thins out towards its fellow of the 
opposite side, which it does not quite meet. 
Section 2.—The next section (Plate 14, fig. 8) is through the widest part of the 
hemispheres (CP) which are nearly oval in the section, from side to side, the top being 
convex as well as the bottom ; this latter part is, however, narrower than the upper. 
