DEVELOPMENT OF THE SKULL IN STURGEONS. 
171 
seen ( p.br .), and outside that the most backwardly projecting part of the massive 
hyomandibular (Jim.). 
Section 19.—The last of this series (Plate 17, fig. 11) is post-cranial; the ganglion 
of a spinal nerve (sp.n.) is cut across ; the medulla spinalis (my.) is seen to he very 
small in proportion to the spinal cavity, and the notochord (nc.) is twice as large as 
in the skull, find reaches the theca vertebralis. The parasphenoid ( pa.s .) is in two 
parts (see Plate 16, fig. 3); the hind and transverse processes of the undivided vertebral 
tract are very large, and are covered with large scutes. This section is behind the 
operculum (Plate 15, fig. 10, op.), and through one of the hinder branchial arches; in 
the general branchial recess, the pharyngo-, epi-, and part of the cerato-branchials 
(p.br., c.br., e.br.) of a middle gill arch, and some of the gills, are shown. 
Behind this part the notochord becomes thicker, and its chondrified sheath more 
distinct; the forked parasphenoid dies out, and the neuro-central cartilages appear 
(Plate 16, fig. 3). 
Fifth Stage. — Adult Sturgeon (Acipenser sturiojA 
The Sturgeons dissected for this stage were about 4 or 5 feet long, such as are most 
commonly seen in the markets ; my last stage is from a much larger specimen, but 
not from the largest. Dr. Gunther (‘ The Study of Fishes/ p. 362) says that this 
species attains a length of 18 feet. 
The swelling of the fore part of the trabeculae and intertrabecula in the orbital 
region becomes very remarkable, and the cartilage becomes subdivided into an upper 
and a lower stratum; between these the parasphenoid (Plate 18, fig. 3, tr., i.tr., pa.s.) 
grows as a thin dentate lamina, with one very long median spike. The clavate swell¬ 
ings of the trabeculae bind on the sides of the exposed part of the parasphenoid, and 
the second floor of cartilage in the middle (i.tr.) runs up to the base of the huge rostrum, 
appearing below as a rounded elevation, separated from the lateral lobes by a deep 
sulcus, which is filled with fatty tissue. In the fatty tissue outside the paired lobes 
of cartilage there is a series of four thin cartilaginous plates (e.pa/.) ; they are outside 
the trabeculae, and behind the ethmo-palatine boundary of the nasal sac, and probably 
represent, partly, as Mr. Howes suggests, the free ethmo-palatines of other kinds; in 
Siredon I have shown that there are two of those cartilages on each side (Phil. Trans., 
1877, Plate 24, figs. 1-3, e.pa., pt.pa .)—an te ethmo-palatine ” and a “ post-palatine.” t 
* The dissections, figures, and notes for this stage were made for me by my friend Mr. George Howes, 
Professor Huxley’s talented Demonstrator. 
f 1 wish to put all these pre-oral growths together for comparison, in hope of obtaining a clue to 
their real nature. Mr. Balfour’s writings have made me very doubtful of the view that, at one time, 
seemed to me to be almost demonstrable—viz., that these antorbital cartilages were in reality the 
rudimentary upper part of a pre-mandibular visceral arch , but their position, in front of the hypoblast, 
makes their determination as difficult as that of the pro-chordal part of the basis cranii, 
Z 2 
