DEVELOPMENT OF THE SKULL IN STURGEONS. 
141 
Pavesi, Dr. Pietro. ‘Seconda Contribuzione alia Morfologia e Sistematica dei 
Selachi.’ Pp. 1-73; plate 3. Genoa: 1878. 
Salensky, Professor W. 6 On the Development of the Sturgeon ( Acipenser ruthenus).’ 
Kasan: 1878. 
Traquair, Dr. B. H. 
1. “ On the Cranial Osteology of Polypterus.” Jour, of Anat. and Phys., vol. v., 
pp. 166-182; plate 6. 
2. “ On the Ganoid Fishes of the British Carboniferous Formations.” Trans. 
Palmont. Soc., 1877 (Part I.: Palseoniscidse), pp. 1-60 ; plates 1-7. 
3. “ On the Structure and Affinities of the Platysomidse.” Trans. Boy. Soc. 
Edinb., 1879, vol. xxix., pp. 343-391 ; plates 3-6. 
Wiedersheim, Dr. Bobert. ‘ Morphologische Studien/ Part III. : Das Skelet und 
Nervensystem von Lepidosiren annectens (Protopterus , ang.) Plates 2 and 3. 
Jena : 1880. 
First Stage. — Newly-hatched embryos of Acipenser ruthenus from millims. 
to 6^ millims. in length. 
The specimens figured by me (Plate 12, figs. 1-3) are rather smaller than the one 
figured by Mr. Balfour (‘Comp. Embryo!,’ vol. ii., p. 88, fig. 53) which measured 
7 millims. in total length, but his description may serve for mine, which were nearly 
as much developed as his slightly larger specimen. 
After describing the development of the embryo, Mr. Balfour explains its peculiar 
outspread form in its unhatched condition (op. cit., p. 86, fig. 52), and says:—“The 
further changes which take place are, in the main, similar to those in other Ichthy- 
opsida, but in some ways the appearance of the embryo is, as may be gathered from 
fig. 52, rather strange. This is mainly due to the fact that the embryo does not 
become folded off from the yolk in the manner usual in the Vertebrates; and as could 
be shown in the sequel, the relation of the yolk to the embryo is unlike that in any 
other known Vertebrate. The appearance of the embryo is thus something like that 
of an ordinary embryo slit open along the ventral side and then flattened out. Organs 
which properly belong to the ventral side appear in the lateral parts of the dorsal 
surface. Owing to the great forward extension of the yolk the heart (fig. 52, B) 
appears to be placed directly in front of the head.” Then, after describing the 
progress made in the development of the brain and organs of special sense, Mr. 
Balfour says :—“ At the sides of the cephalic plate the visceral arches make their 
appearance, and in fig. 52, A and B, there are shown the mandibular (Md.), hyoid (Fla.), 
and first branchial (Br.) arches, with the hyo-mandibular (spiracle) and hyo-branchial 
clefts between them. They constitute peculiar concentric circles round the cephalic 
plate, their shape being due to the flattened form of the embryo already alluded to.” 
And then, further on (p. 88) our author says “ Before hatching, the embryo has to 
