46 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
in this reconstruction, is seen only at the beginning of the sagittal section in the region. It is still attached 
to the roof of the archenteron at this point. 
Fig. 4. Graphic reconstruction of an embryo 2.7 mm. in length. Normal plate. series No. 9. 
(H.E.C. 097.) °X 50. 
This is another reconstruction of the embryo illustrated in the preceding figure. All structures, 
except the notochord, are sectioned along the median line, so that one sees the right side of the inner 
surfaces of the medullary groove and archenteron. The former structure shows points of interest not 
mentioned in the description of Fig. 3. The fore-gut portion of the archenteron extends forward to 
the cephalic plate, and from its base the preoral gut projects forward between the optic vesicles. 
Cp. No. N.9. 
In .neur. 
P.mes. P.g. 2 Gl.p. * A. Yolk 
Fig. 4. 
The first gill pouch is represented by an irregular lateral expansion of the fore-gut (Gl.p.). The arch- 
enteron posterior to the fore-gut is floored in part by irregular masses of cells; and these, where they cross 
the median line, are represented by the areas shaded in oblique lines (*, *). A large mass of preoral 
mesoderm has proliferated from the anterior wall of the archenteron, and is represented in the reconstruction 
as sectioned along the median line. It is shaded in broken oblique lines. So far as the notochord is a 
separate structure, it is shown as a rounded rod. At the anterior and posterior ends, where it fuses with 
the preoral mesoderm and the roof of the archenteron respectively, the notochord is cut along the median 
line. As indicated by the shading, all the germ layers become confluent in the anterior wall of the 
incisura neurenterica (In.neur.). 
Fig. 5. Graphic reconstruction of an embryo 3.5 mm. in length. Normal plate series No. 14. 
(EREIOEOS2H) E50: 
The skin ectoderm is represented as cut away dorsally, a little to the left of the median line. At 
the anterior end of the head, the cut passes a little behind the large anterior neuropore (An.neur.), and then 
along the ventral median line of the head and pharyngeal region. The posterior end of the embryo is cut 
along the mid-sagittal line exposing the right internal surface of the medullary groove (M.g.) and incisura 
neurenterica (In.neur.). Both entoderm and mesoderm are cut away along the line where they flatten out 
upon the yolk. 
The cephalic end of the medullary tube shows a marked ventral flexure. The optic vesicle (Op.v.) 
projects outward and backward from the expanded fore-brain. Above this region, and overlying the upper 
edge of the medullary tube, is a narrow cephalic crest (C.ceph.) which is represented in the figure in stipple. 
Although its ventral edge is extremely irregular, there is no division into separate nerve anlagen. The 
folds over the chordal portion of the medullary tube are fused from the hind-brain region to a point a 
little anterior to the beginning of the median sagittal section shown in the figure. 
