Normal Plates of the Development of Squalus acanthias, 53 
nerve ganglia. The two ganglia of the spino-occipital nerves (@.sp.oc.) are represented by enlargements of 
the dorsal ganglionic commissure. The ventral roots of the two spino-occipital and the anterior fifteen 
spinal nerves are established. 
The pharynx (Ph.) is somewhat flattened dorso-ventrally. From its anterior end projects the posterior 
portion of the hypophysis (Hyp.). The anterior portion of this structure is as yet not invaginated. The 
two anterior gill slits (G1.s.I, II) are open, and the ectoderm and entoderm are confluent over the third and 
fourth pouches. In the figure, the skin ectoderm is represented as left in position over these fused areas. 
The oesophagus (Oes.) is long and slender. The liver (L.) arises from the gut at the juncture of the 
oesophagus and the vitelline duct (D.vit.). The pancreas is here seen for the first time in the series of 
reconstructions. It is a small pouch (Pane.) from the dorsal wall/of the gut, over the middle of the vitelline 
duct. The rotation of the gut to the left is already begun. This process has produced a shallow 
groove (L.g.) which extends the entire length of the left wall of the yolk stalk. The spiral valve (Sp.v.) 
at this stage appears as a shallow groove in the wall of the epithelial tube of the intestine. It arises 
on the left side of the yolk stalk, posterior to the pancreas, and makes a little more than one complete 
turn about the gut. The cloaca (Cl.) is somewhat dilated and its floor is in contact with the skin ectoderm 
below it. 
The Wolffian duct (W.d.), although present in the embryos of this series from 3.8 mm. in length 
upward, is here represented for the first time in the series of reconstructions. It has been left unshaded. 
It arises anteriorly from a long groove in the dorsal wall of the coelom and from this groove arise also 
the pronephric tubules. To avoid obscuring the origin of the vitelline artery, the anterior end of the duct 
has been omitted, and the structure is represented as cut across at the lower end of the coelomic funnel 
described above. 
The sinus venosus, atrium (4#.), ventricle (V.), and bulbus cordis (B.cor.) are to some extent 
differentiated. The dorsal mesocardium is entirely absent except at the extreme anterior end of the heart. 
The first and second aortic arches are completely formed. The third is represented by sprouts from the 
dorsal and the ventral aortae, and the fourth by a sprout from the dorsal aorta only. The vitelline 
artery (A.vit.) arises by two trunks (the remains of the two anterior right pronephric arteries) and passes 
ventrally on the right side of the yolk stalk. That part of its course which lies behind the right wall of 
the radix mesenterica (M.mes.) and the yolk stalk is drawn in broken lines. The third pronephric artery 
appears as a slight swelling behind the posterior root of the vitelline artery. The vena capitis medialis 
(V.cap.med.) is still broadly continuous with the dorsal aorta. No distinct vessel can be traced beyond the 
posterior edge of the trigeminal crest, but occasional irregular chambers, connected with the dorsal aortae, 
were present behind this point, in the position occupied later by the posterior part of the v. capitis medialis. 
While it is possible that they represent portions of a partially collapsed but continuous vessel, no such 
structure can be traced in the sections. The left vitelline vein (V.vit.s.) passes from the heart between the 
lobe of the liver diverticulum and the gut, and backward along the groove in the wall of the yolk stalk, 
described above. Near the end of the vitelline duct it receives the subintestinal vein and, passing abruptly 
ventrally, goes to the blastoderm. Anterior to this union with the subintestinal vein, it gives off small 
dorsal shoot which has been termed the recurrent vitelline vein (V.vit.rec.), and which later unites with the 
vitelline vein of the right side. At this stage the vitelline vein of the right side is much smaller than 
that of the left and is not connected with the subintestinal vein. It does not extend to the blastoderm. 
The subintestinal vein (V.subint.) is single throughout the greater part of its course, but bifurcates in front 
of the cloaca. Both right and left branches end blindly after passing a short distance upward on the walls 
of the cloaca. 
