DEVELOPMENT OF THE SKULL IN STURGEONS. 
143 
appear after the third month of larval life. In front of the mouth two pairs of papillae 
grow out, which appear to be of the same nature as the papillae in the suctorial disc 
of Lepidosteus (figs. 54, 55, p. 89). They are very short in the embryo, represented in 
fig. 53. Soon, however, they grow in length (figs. 54 and 55, st.) ” [Plate 12, figs. 4, 5, 
7, 9], u and it is ” [ more than'] “ probable that they become the barbels, since both sets 
of structures occupy a precisely similar position.” 
“ 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.) ” 
[Plate 12, figs. 4, 7, 9, ol.]. “ It is probable that the two openings of each nasal sack, so 
established in these and in other Fishes, correspond to the external and internal nares 
of the higher Vertebrata. At the time of hatching there is a continuous dorso-ventral 
fin” [Plate 12, figs. 1,2], “ which by atrophy in some parts, and hypertrophy in other 
parts, gives rise to all the impaired fins of the adult, except the first dorsal and the 
abdominal. The caudal part of the fin is at first symmetrical, and the heterocereal 
tail is produced by the special growth of the ventral part of the fin combined with an 
atrophy of the dorsal part.”'" 
Referring the reader to Mr. Balfour’s work and to my own plates, I shall now 
simply describe the figures of the general form of the first stage, and then those of the 
second and third, in which also will be given a detailed account of their skeletal 
structures. 
The fourth, fifth and sixth stages will be of the other species (A. sturio), and that 
in a thoroughly metamorphosed condition. 
The smallest of my specimens (5^ millims. long, Plate 12, fig. 1) is intermediate 
between figs. 52 and 53 of Mr. Balfour (op. cit. , pp. 87, 88); the original of his fig. 52 
(A and B), may be found in Professor Salensky’s Russian work (plate 7, figs. 56 and 
58), whilst his fig. 53 is only slightly larger than my second specimen (Plate 12, 
figs. 2 and 3). 
In the smallest specimen, scarcely hatched at the time when it was put into the 
spirits, the large hind-brain (C 3 .) and the lesser mid- and fore-brain (C 3 ., C 1 .) are seen 
arranged in an accurate series. The auditory sac (au.) is below and behind the wide, 
front part of the hind-brain, the eye (e.) below and between the fore- and hind-brain, 
and the olfactory sac (ol.) on the side of and below the fore-brain, in front of the eye. 
The mouth (m.) is a considerable rhomboidal space on the ventral aspect of the head, 
and behind it we see three folds, two clefts, and a general hollow space behind the 
third fold. These folds are the rudiments of the mandible, hyoid, and first branchial 
(mn., hy., hr 1 .), the clefts are the hyomandibular (cl 1 .) and the hyo-branchial (cl 2 .), and 
the fossa behind leads to the tissues that are preparing to form the rest of the 
branchial arches with their intervening clefts. The last third of the spinal region is 
* I am glad to be able to give, and the reader will not be sorry to have, an account of these early laryge 
in Mr. Balfour’s own words. Of course I have been over the same ground, but my work has been made 
much easier by my friend’s kindness in sending me his proofs as soon as they were in type. 
