66 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
which are illustrated in preceding reconstructions. The stalk of the mandibular somite (So.mand.) is still 
more elongated as compared with the expanded distal extremity. The distal half of the premandibular 
Ourir. So.hyo. C.fa.ac.  C.g-ph. Crvag. somite (So.prem.) lies mesial to the mandibular 
somite.e The premandibular somite is also 
elongating antero-posteriorly and its posterior 
extremity now extends into the upper part 
of the first gill arch. The posterior edge of 
ur. the anterior somite (So.ant.) is now slightly 
overlapped by the anterior part of the pre- 
mandibular. All the walls of the anterior 
somite are now definitely outlined in meso- 
Pe. thelium. The somite has shifted in position 
Op. ur | So.mand. 1st@l.c. St.mand. St.hyo. until its long axis lies almost exactly in the 
o.prem. 
Fig. 19. transverse plane of the body. 
Fig. 20. Reconstruction of the head somites of an embryo 9 mm. in length. Normal plate series 
No. 23. "(ERE:C 27495.) 7%,33: 
This figure represents the head cavities of an embryo already shown in reconstruction in Fig. 11. 
The mandibular somite (So.mand.) is very much elongated and is attached by a stalk nearly four times as 
long as the somite itself, to the anterior end of the pericardium. The premandibular somite (So.prem.) is 
lenticular in cross section, and is covered anteriorly by the mandibular and anterior somites. The anterior 
somite is crescenteric in outline, and lies 
N.trig. N.fa.ac. Oto. D.endol. .g-ph, 
.g-ph. & i ö 
een a Comm.d. in a small space between the optic vesicle 
— 
below and the mandibular somite above. 
Its anterior portion lies almost exactly in 
the antero-posterior plane of the body. The 
C.urtr. 
hyoid somite (S$o.hyo.), which is completely 
‚So.mand. f f 
separated from its stalk, overlaps the mandi- 
FT bular somite a little dorsally. The stalk of 
So.ant. 
the hyoid somite (St.hyo.) extends upward 
from the pericardium, from which it 
originates in company with the stalk of 
the mandibular somite, to the top of the 
1st@l.c. St.mand. St.hyo. Pew. 
- hyoid arch. Its dorsal end is mesenchymous. 
Fig. 20. 
The first, second and third branchial somites 
have not been represented in the reconstruction, but their stalks which are in the main mesenchymous, 
are indicated as cut off close to their connection with the pericardium. The general anatomy of the embryo 
is described in connection with Fig. ı1. 
Fig. 21. Dissection of an embryo of Squalus acanthias, 15 mm. in length, corresponding to normal 
plate series No. 26. X 6. 
The following is the first of a series of dissections illustrating the general appearance of the viscera 
of a series of embryos carefully correlated with certain stages of the normal plate series. The ventral body- 
