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48 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
The first two gill pouches (Gl.p.I, II) appear as shallow depressions in the right wall of the fore-gut. 
Behind the fore-gut the archenteron is floored in part by a mass of entodermal cells (*), such as was 
described in connection with the reconstruction of an embryo 2.7 mm. in length (Fig. 4). The same method 
of shading is employed as in that figure. The notochord (No.) is extremely irregular in shape. Its anterior 
end is fused with the preoral mesoderm and the posterior end merges into the general cell mass in front 
of the incisura neurenterica (In.neur.). 
Fig. 7. Graphic reconstruction of an embryo 4.0 mm. in length. Normal plate series No. 16. 
(H.E.C. 930.) X 33. 
In this reconstruction, all structures of mesodermal origin have been omitted, with the exception of 
the preoral mesoderm which lies between the anterior wall of the gut and the floor of the brain, and which 
is sectioned at the median line. The skin ectoderm is cut along the median line except in the region of 
the anterior neuropore, where the line of section passes a little to the left. As in preceding reconstructions, 
the tail region is sectioned along the mid — sagittal line, showing the right half of the internal surface of 
the walls of the gut, medullary canal and neurenteric canal. 
O.gph.-vag. No. Snr. M.r. 
Oto.d. x - a, - = | 
C.fa.ac. S I 1 Neur.c. 
UN 
ZEDDEEZER 
C:thal. 
Gl.p. I. Gl.p. II. Oes. 
Op.v. 
Ani.neur. Rec.postop. Or.pl. Fig. 7. 
The fore-brain vesicle is bent downward and a little backward from the mıd-brain. From its base 
projects a large shallow pouch, the primitive infundibulum or recessus postopticus (Rec.postop.). The 
optic vesicle (Op.v.) is broadly connected with the ventral zone of the fore-brain and extends forward and 
upward, overlapping the dorsal half of the lateral walls. There is a small anterior neuropore (Ani.neur.) 
the walls of which are almost in contact. The skin and neurectoderm are fused along the median line 
both posterior and anterior to it. A shallow groove separates the fore-brain and mid-brain, and a less 
marked one the mid-brain and hind-brain. A slight depression in the wall of the hind-brain indicates the 
position of the otocyst (Oto.d.). 
The cephalic crest, which is indicated in stipple, is divided into two parts. The anterior division, 
which extends from the optic vesicles backward to the hind-brain, represents the anlagen of three nerve 
crests: the thalamic, facial-acoustic, and trigeminal. The thalamic crest (C.thal.) is represented by a small 
expansion above and posterior to the eye. The trigeminal and facial-acoustic crests (CO.irig., O.fa.-ac.) are 
still confluent, but two small interruptions indicate the line of division between them. The posterior segment 
of the cephalic crest (C.g-ph.-vag.) represents the anlagen of both the glossopharyngeal and vagus crests, but 
as yet there is no differentiation of these two elements. 
The archenteron is divided into fore-, mid- and hind-gut. The fore-gut consists of an expanded 
anterior portion, the pharynx (Ph.), and a short flattened oesophagus (Oes.). The floor of the anterior 
end of the pharynx is in contact, but not fused, with the thickened ectodermal portion of the oral plate 
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