142 
MR. W. Iv. PARKER ON THE STRUCTURE AND 
a small extent become folded off from the yolk both anteriorly and posteriorly ; and 
has also become to some extent vertically compressed. As a result of these changes, 
the general form of its body becomes much more like that of an ordinary Teleostean 
embryo. The general features of the larva after hatching are illustrated by figs. 53, 
54, and 55. Fig. 53 represents a larva of about 7 millims., and 54 a lateral and 55 a 
ventral view of a larva about 11 millimsA There are only a few points which call for 
special attention in the general form of the body. In the youngest larva figured ” 
[fig. 53, 7 millims. long ; and see also Plate 12, figs. 1-3 of the present paper of some¬ 
what younger larvae] <£ the ventral part of the hyomandibular cleft is already closed : 
the dorsal part of the cleft is destined to form the spiracle (sp.). The arch behind is 
the hyoid, on its posterior border is a membranous outgrowth, which will develop 
into the opercular membrane. In the older larvae a very rudimentary gill appears to 
be developed on the front walls of the spiracular cleft, but I have not succeeded in 
satisfying myself about its presence; and rows of gill papillae have appeared on the 
hyoid, and the true branchial arches (figs. 54 and 55, g).” [The mandibular gill, about 
which Mr. Balfour speaks doubtfully, is according to my observations a thickened 
mass of hypoblastic cells lining the front wall of the first or “ spiracular ” cleft. This 
mass is crescentic (Plate 12, figs. 2, 4, and 7, cl 1 .), and is slightly grooved along its 
hinder margin; on each side of this groove the low ridges are imperfectly divided into 
little rounded masses, which appear to me to be, evidently, the rudiments of branchial 
papillae; at the lowest computation, this soft, tuberculate mass of cells is homologous 
with the tracts of cells which do develop into branchial papillae in the Elasmo- 
branchii.] “ The biserially-arranged gill papillae of the true branchial arches are of con¬ 
siderable length” [see Plate 12, figs. 4-9], “and are not yet covered by the operculum, 
but they do not form elongated thread-like external gills like those of the Elasmo- 
branchii. The oral cavity is placed on the ventral side of the head ; it has a more or less 
rhomboidal form. It soon however (fig. 55) becomes narrowed to a slit with projecting 
lips, which eventually becomes converted into the suctorial mouth of the adult. The 
most remarkable feature connected with the mouth is the development of provisional 
teeth (fig. 55) on both jaws. These teeth were first discovered by Knock (‘Die 
Beschr. d. Beise z. Wolga Behufs. d. Sterlettbefrucfcung,’ Bull. Soc. Nat., Moscow, 
1871). They do not appear to be calcified, and might be supposed to be of the same 
nature as the horny teeth of the Lamprey. They are, however, developed like the 
teeth, as a deposit between a papilla of subepidermic tissue and an epidermic cap. 
The substance of which they are formed corresponds morphologically to the enamel of 
ordinary teeth. As they grow they pierce the epidermis and form hollow spine-like 
structures with a central axis filled with subepidermic mesoblastic cells. They dis- 
* These do not correspond precisely to the specimens worked ont by me, and of which I also have given 
figures illustrating the outward form. My first Stage includes larvse from 5^ millims. to 6^ millims. in 
total length; Stage two, 8| millims. to 9| millims.; and Stage three, 13| millims. to 14 \ millims. The 
largest larva of this species in those sent to me was 14| millims. (x^ths of an inch) in total length. 
