Normal Plates of the Development of Squalus acanthias. 63 
and distinctly marked off from both the oesophagus and the duodenum. The duodenum (Duo.) is of large caliber, 
and is directed downward and backward at an angle of nearly forty-five degrees to the stomach. At its base 
it receives the ductus choledochus (D.chol.). The vitelline duct (D.vit.) now much reduced in size, enters the 
gut at the point of juncture of the duodenum with the intestine proper. The twisting process which forms 
the spiral valve has also produced a deep furrow on its posterior and ventral surface. The spiral valve ($p.v.) 
makes thirteen complete turns. The segment of the gut lying immediately behind the spiral valve and 
anterior to the cloaca tapers gradually posteriorly. The cloaca (Ül.) receives the Wolffian ducts. Its floor 
is in contact along its entire extent with the anal plate of ectoderm. The liver (L.) has been represented 
in median section as in the reconstruction of the embryo of 18 mm. (Fig. 15). The gall bladder (G.bl.) now lies 
with its long axis almost in the dorso-ventral plane of the body. It is expanded distally, and constricted 
at its upper end to form a short broad cystic duct (D.cyst.). This structure opens into a small chamber, 
corresponding with the original median chamber of the liver, which receives also three hepatic ducts. From 
this median chamber the ductus choledochus (D.chol.) passes backward to the duodenum first bending 
upward and then downward in its course. The pancreas (Panc.) is now broadly expanded distally, and a 
large number of secondary pouches are given off from the walls of the original one. The large pancreatic 
duct (D.pane.) is represented by a ridge upon the lateral surface of the structure. The duct opens into 
the intestine on the ventral edge of its left surface just posterior to the duodenum. Near the entrance 
of the pancreatic duct is a small mass of pancreatic tissue which represents the head of the adult pancreas 
Be This is the first figure showing the rectal or digitiform gland (Dig.gl.). This structure arises 
from the right and dorsal surface of about the middle of the post-valvular portion of the gut, and extends 
a little forward. It is connected with the gut along almost its entire base. Anterior to the digitiform 
gland is a long narrow slit in the mesentery, the beginning of the mesenteral fenestra (Mes.f.). The 
Wolffan duct (W.d.) arises by a long coelomic funnel at the level of the sixth spinal nerve, and joins 
the cloaca near the dorsal median line and its posterior third. Thirty-seven mesonephric tubules are 
present, but only the anterior five are represented in the reconstruction, the others being indicated only 
by the ends of their distal portions where joining the Wolffan duct. The first three mesonephric tubules 
are rudimentary, the first two not being fused with the Wolffian duct. The fourth and fifth mesonephric 
tubules (Mes.t.) show the characteristic form of the series, and consist of a narrow anterior and a dilated 
posterior chamber, the latter communicating with the Wolffian duct by a constricted neck. These may be 
compared appropriately with those represented in Figure 20 of Plate XVII of Carr Ragr’s “Theorie des 
Mesoderms”. 
The outlines of the heart are somewhat obscured by the overlapping blood vessels. The general 
antero-posterior shortening of the structure is, however, quite noticeable. The bulbus cordis is very 
much shortened and thickened and tapers down to a vessel of small caliber anteriorly. The formation of 
the efferent and afferent branchial vessels is now far advanced. In the reconstruction the afferent series of 
vessels is differentiated from the remaining arterial system by being represented in black crossed with white. 
The original ventral connection of the first aortic arch with the ventral aorta is now completely obliterated, 
but extremely small vascular cords represent the position of the connection. There is no indication 
of a thyreo-spiracular artery in this specimen. A broad and well established vessel now connects the 
hyoidean artery with the first aortic arch. Midway between this anastomosis and the sinus cephalicus the 
capillary system of the spiracular demibranch (*) is in the process of formation. The posterior part is already 
differentiated into short vascular loops the cut ends of which are seen in the figure. The anterior part is 
in a state of development comparable with that seen in the second and third arches in the embryo of 
15 mm. (Figs. 13, 14). The afferent artery of the hyoid arch is completely separated from the efferent 
