T|—— 
Normal Plates of the Development of Squalus acanthias. 65 
and it has been found impossible to determine the exact boundary between its anterior wall and the 
mesectoderm of the thalamic nerve crest. The approximate boundary of the two structures is indicated by 
a dotted line. The hyoid somite (So.hyo.) lies above So.mand. So.hyo. O.g-ph.-vag. 
the dorsal margin of the distal expansion of the 
DD 
C.trig.fac. 
III 
S 
mandibular somite. It is still broadly connected with 
IE 
the lateral mesoderm posteriorly, and with a short 
broad stalk which passes downward and unites with 
the stalk of the mandibular somite at the base of 
the hyoid arch. No definite pericardial cavity has 
been established at this stage, but it will be noticed 
that the ventral portions of the lateral plate of St.mand. St.hyo. 
mesoderm (Pe.w.) are extended laterally on either side 
of the anterior intestinal portal. Between the inner Op. So.ant. So.prem. 
surface of this portion of the lateral mesoderm, Bsız 
and the lateral walls of the digestive tube is found an irregular and broken cord of angioblastic cells, — 
the anlage of the endothelial heart. 
Fig. 18. Graphic reconstruction of the head cavity of an embryo 4.8 mm. in length. Normal plate 
series No. I8. (H.E.C. 1398.) X 33. 
For general anatomy this reconstruction may be compared with Fig. 8, a reconstruction illustrating 
the general anatomy of the same embryo. The mandibular somite (So.mand.) now lies somewhat more 
anteriorly and dorsally in relation to the brain than it did in the preceding reconstruction. Its stalk 
is somewhat longer and its ventral and posterior part is curved slightly downward; a change due to 
the rapid growth of the first gill arch. Only 
C.trig. 
the dorsal portion of the premandibular somite 
(So.prem.) now lies mesial to the mandibular. N ne 
The anterior somite (So.ant.) is now completely Ciihal. 
separated from the premandibular somite, although 
it is nearly in contact with that structure. It lies 
entirely anterior to the premandibular, as well as 
. 3 { i 2 en 
the mandibular, somite. Anteriorly and ventrally u e) 
it becomes indistinguishable from the mesectoderm Opw. So.ant. | So.mand. 1st@l.c. St.mand. Pe.w. Pe. 
: . o : So.prem. 
of this region, in the same manner as in the Es 
ig. 18. 
embryo of 4.0 mm. The hyoid somite ($o.hyo.) 
now lies mainly above and a little anterior to the first gill plate. Although broadly connected with the 
mesoderm of the second gill arch, a slight constriction in its ventral border indicates the point at which it 
will later separate from this mass. 
Fig. 19. Graphic reconstruction of the head somites of an embryo 5.2 mm. in length. Normal 
plate series No. 20. (H.E.C. 1352.) X 33. 
This is a reconstruction of the same embryo as that represented in Fig. 9. The elongation of the 
pharyngeal region and general growth of the head has continued the changes in position of the somites 
Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. XII. 9 
