30 Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 
Urogenital System 
Heart, Pericardium, Blood Vessels 
21 
22 
23 
24 
25 
26 
ı Wolffian duct attached to three anterior pronephric 
anlagen, extends posteriorly for 73 sections of 6 u. 
as a solid cord of cells. Fused with ectoderm for 
last fourth of its length. 
Four anterior pronephric tubules possess lumina and 
are connected with Wolffian duct. A minute lumen 
in Wolffian duct just posterior to third pronephric 
tubule — all other parts solid. Wolffian duct ex-| 
tends backward nearly to anterior end of cloaca. 
|Wolffian duct connected with coelom by two pro- 
nephric funnels. A small lumen in Wolffian duct 
for 80 u (by sections) posterior to last pronephric 
funnel; posterior to this point duct is solid. Duct 
ends blindly at anterior end of cloaca. 
Five pronephric tubules, first separate, remainder 
forming canals connecting coelom with Wolffian duct. 
Lumen present in anterior part of Wolffian duct, 
about five sections of 8 „only. Wall of duct thickest 
dorsally and mesially. About Iz nuclei seen in cross 
section. Duct extends posteriorly as far as extreme 
anterior end of cloaca, where it lies in contact with 
the ectoderm. Eight pairs of mesonephric tubules 
| clearly established. They are broadly continuous 
| with coelomic epithelium, and not in contact with 
| Wolffian duct. 
Wolffian duct extends backward as far as middle of 
|eloaca; turned sharply downward at its extreme 
posterior end, and in contact with cloacal epithelium. 
A lumen present except in extreme posterior end. 
Twenty-six pairs of definite mesonephric tubules. 
e 
Anterior end of Wolffian duct at level of vitelline 
artery. Posterior end fused with lateral wall of 
cloaca. Twenty-nine pairs of mesonephric tubules. 
Wolffian duct opens anteriorly into coelom, just in 
front of pancreas. Posteriorly it is fused with wall 
of cloaca. Forty pairs of mesonephric tubules, the 
anterior three rudimentary. Tubules curved over 
dorsal surface of Wolffian duct. Interrenal organ a 
mass of condensed mesenchyma lying between aorta 
and radix mesenterica, extending from level of first 
turn of spiral valve to cloaca. 276 germ cells in 
genital region (WOODS). 
Posterior part of heart is strongly bulged dorsally and to the right. Anteriorly, dorsal mesocardium long 
and thin. Broken through over middle part of heart for 30 sections of 6 u. Mesothelial layer of heart a 
little thicker than in preceding stage, being made up of cuboid or columnar cells. — Upper end of first 
aortic arch dilated (anlage of arteria ophthalmica magna). Vessels from dorsal and ventral aorta almost 
meet to form second arch. Pronephric arteries represented by three dilatations of the aorta on the right 
side in pronephric region. — Veins as in preceding stage (No. 19). 
Dorsal mesocardium only present at anterior and posterior ends of heart tube. — First and second aortic 
arches complete. The three pronephric arteries of right side united, forming vitelline artery which extends 
to blastoderm. 
Heart shows slight differentiation into atrium, ventricle, and bulbus. Walls of ventricular chamber slightly 
thickened. Dorsal mesocardium attached only at posterior and anterior ends of heart. Somatic mesoderm 
and splanchnic mesoderm !in contact above anterior ends of. vitelline veins. — First two aortic arches 
complete. Branches from dorsal and ventral aortae to form third and fourth arches. Dorsal aorta extends 
backward as far as cloaca. It is a single vessel from level of fourth pharyngeal pouch backward — 
double at its extreme posterior end. Vitelline artery arises from two trunks. A small dilatation of dorsal 
aorta, posterior to second root of vitelline artery, represents third pronephric artery. — Anterior cardinal 
complex, as described for preceding stage (No. 21). Subintestinal vein single to cloaca. Left vitelline 
vein larger than right, connects with single subintestinal vein posteriorly. Right vitelline vein extends 
backward only to level of vitelline artery, no connection with subintestinal vein. A few corpuscles in 
the heart and dorsal aorta — irregular cells with large oval nuclei, containing one or two dark staining 
chromatin masses. 
Heart somewhat more twisted than in preceding stage. Ventricle lies below and to the left of atrium. 
Slight constriction of atrio-ventricular canal. A slight constriction also between ventricle and bulbus. 
Mesothelial wall of sinus venosus is fused for a short distance with the mesothelium of somatopleure. — 
First, second, and third aortic arches completely formed. Fourth and fifth represented by short sprigs. 
Dorsal aorta extends backward beyond cloaca. Vitelline artery arises from dorsal aorta by two roots of 
about equal size. — Vena capitis medialis connected with aorta as in preceding stages. Extends forward 
to dorsal surface of optic cup, and backward beyond posterior edge of vagus; at this point turns sharply 
ventrally, joining with anterior cardinal at level of aorta. Anterior cardinal extremely irregular, connected 
by numerous channels with aorta. Joins with posterior cardinal, forming common cardinal, at level of 
second spinal ganglion. Common cardinal extremely irregular; ends in capillary network in somatopleure, 
lateral to posterior end of heart; does not enter heart. Posterior cardinal can be traced backward a few 
sections only from juncture with anterior cardinal. Vitelline veins much as in preceding stage (No. 22). 
A recurrent branch from subintestinal and left vitelline vein passes dorsally to upper surface of gut, and 
thence forward to the right of the pancreas until almost meeting right vitelline vein. Subintestinal vein 
bifurcated posteriorly; its branches form a loop around cloaca, meeting on its dorsal surface to form a 
short caudal vein. 
Ventricle lies mainly ventral to atrium. Atrio-ventricular canal connects anterior ends of atrium and ventricle 
— it descends directly downward and is sharply constricted. Sharp constriction also between sinus 
venosus and atrium. Splanchnic and somatic mesoderm lying on either side of sinus venosus fused for 
200 u. antero-posteriorly (by sections). — Four aortic arches completely formed, fifth in process of formation. 
Aorta much dilated in region of fifth arch, bifurcated from level of posterior end of vagus forward. Two 
roots to vitelline artery, first root three times diameter of second. — Vena capitis medialis dilated above 
optic cup to form orbital sinus. Lies mesial to trigeminal, facial, glossopharyngeal, and vagus nerves, and 
otocyst. Communicates with anterior cardinal proper by two channels at level of vagus. Three segmental 
veins connect with anterior cardinal behind this junction. Common cardinals lie at level of solid oeso- 
phagus. Posterior cardinals extend backward to twentieth mesonephric tubule. Right and left vitelline 
veins communicate, posterior to pancreas. Right vitelline vein about half the size of left. 
Heart much as in preceding stage (No. 24). — Five aortic arches. Vitelline artery arises from two roots, 
anterior root slightly the smaller. Anterior cardinals and vitelline veins similar to those of preceding 
embryo (No. 25). Posterior cardinals lie mesial to Wolffian ducts: become extremely irregular posteriorly. 
Cannot be traced beyond thirtieth mesonephric tubule. 
Heart much elongated, ventricle projects far posterior to atrium. A constricted neck between atrium and 
sinus venosus. Walls of ventricle and bulbus much increased in thickness. Pericardial cavity closed from 
coelom dorsally, except for a narrow slit 230 u. long (by sections) lying to either side of the posterior part 
of the oesophagus. — Five anterior aortic arches complete. Dorsal and ventral sprouts almost meet to 
form sixth. First aortic arch much reduced in size at its base, but dilated below first gill slit. Small 
sprouts from upper ends of second, third, and fourth arches represent anlagen of afferent branchial arteries. 
Each sprout divides into a ventral and a dorsal limb. Capillaries from ventral limb of afferent vessel of 
second arch extend into till filaments. 
