258 MR. A. K. TOTLON ON 
Fig. 9 is practically a lateral view and fig. 10 a dorso-lateral 
view. It will be seen that the posterior neural arches are much 
further developed than the anterior ones. 
Reference to the adult fish (fig. 12) shows that the neural arch 
has, on each side, two points of attachment to the centrum, 
between which points the segmental nerve emerges. Hxamination 
of Stage IX proves that the primary point of attachment is the 
anterior one; the process of formation of the posterior one—as a 
backwardly directed spur (sp.)—can be seen in figs. 9 and 10. 
A comparison of these backwardly directed spurs with some 
cartilages described by Schauinsland in Hertwig’s book (p. 467) 
shows that it is probable that they represent intercalaria. 
Schauinsland says: “The bulk of the base of the arch (carti- 
laginous stage) is at the cranial end of the vertebra, but this base 
sometimes extends caudalwards (Pike). The caudal part may 
even be cut off to form a separate piece of cartilage (e. g. in the 
tail of the Trout). This,” he considers, “ should be looked upon, 
probably in the case of the Pike and pretty certainly in that of 
the Trout, as the remnant of the second arch and as homologous 
to the cranial arch of Amia” (i. e. it is formed by the cranial half 
of a sklerotome: for this reason it is on the caudal end of a 
vertebra). 
Stage X.—The posterior edges of the three large hypurals 
(fig. 11, 4:5 em.) have grown considerably, owing to additions from 
the large enveloping cap of skleroblasts (not shown in figure) on 
which the lepidotrichia are seated. The proximal ends of these 
hypurals have grown also with the notochord, so that a large gap 
(g, figs. 9-11) has appeared—and continually increases in size— 
between the dorsal and ventral hypurals. It will be noticed 
that the posterior neural arch is dividing into two. 
The adult specimen I dissected was 16°5 cm. long. The centra 
are very ‘‘ papery,” and constrict the notochord only very slightly. 
The condition of the posterior neural arch is interesting. At 
first sight the penultimate centrum appears to be carrying two 
arches. Comparison with previous stages, however, shows that 
this is not so, but that the appearance is due to the fact that the 
division, incipient in Stage X, has now been completed. As a 
result of this, the segmental nerve, which in Stage 1X emerged 
beneath the undivided arch, now lies in the cleft, and a secondary 
ossification beneath the nerve has finally reunited the two half- 
arches at their points of attachment to the centrum. Similarly, 
the hypural carried by this centrum has divided, and the two 
halves are covered by confusing secondary ossifications. 
The double neural arch in Plearagramma antarcticum and the 
arch immediately in front of it are considerably more expanded 
and better ossified than those in front of them. 
The posterior part of the hypural borne on the penultimate 
centrum of Pleuwragramma antarcticwm has a thick, well-ossified, 
posterior edge, especially thick at the tip, while the rest of it 
is very thin and membranous. The hypural behind the one just 
