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indicated in black. The upper part of the first aortic arch (Ao.a.I) 
forms an irregular, dilated chamber, from the anterior wall of which 
arise a pair of sprouts (V.cap.med.) which bifurcate almost immediately 
into anterior and posterior branches. These represent the anterior 
end of the vena capitis medialis. The second aortic arch (Ao.a. II) 
is not complete but is represented by branches from the dorsal 
and ventral aortae. 
The notochord (No.) is bent ventrally at its anterior end. 
Below it, from the level of the vagus crest backward, is seen the 
subnotocordal rod (Sn.r.). 
Fig. I0. Graphic reconstructian of an embryo 7.5 mm. in 
length. Normal plate series No. 22. (H.E.C. 1503.) 
x 33. 
The general plan of reconstruction is similar to that of Figs. 8 
and 9, with the exception that the veins and the Wolffian duct, 
which is left unshaded, are included. 
There is no longer any connection in the neuropore region 
between the skin and neurectoderm. The optic vesicle (Op.v.) is 
now indented, forming the optic cup, and in the concavity thus 
formed lies the pear-shaped lens (Ls.). The lens is still broadly 
attached to the skin ectoderm, which is represented as cut away 
around the attachment. There is a deep lentic pit (Ls.p.). A shallow 
infundibular sac projects from the floor of the fore-brain. The 
thalamic (C.thal.), the urtrochlear (C.urir.), and the trigeminal 
(C.trig.) crests are now completely separated. The first and second 
of these are much reduced in size. Two processes project from 
the anterior part of the trigeminal crest. The dorsal process (Pr.urt.) 
is the remnant of the former connection between the urtrochlear 
and trigeminal crests. The ventral process (G.mes.) is the anlage 
of the mesocephalic ganglion. The lower edge of this ventral 
process is expanded over the optic vesicle and is confluent with 
the mesenchyma of this region. The mandibular process of the 
trigeminal crest cannot be traced beyond the upper end of the 
mandibular arch. The acoustic portion of the facial-acoustic crest 
(O.fa., O.ac.) extends a little over the lateral surface of the otocyst. 
The otocyst (Oto.) is connected with the skin ectoderm by a short 
broad duct from which a shallow groove extends upward and 
backward. The glossopharyngeal crest (C.g-ph.) shows no features 
of special interest. The first and second branchial processes of 
the vagus crest are clearly established. From the posterior border 
of the second, a narrow band of cells extends backward above 
the pharynx. The spinal nerve crest extends backward to the 
level of the cloaca. There are twenty-eight anlagen of spinal 
hr 
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