218 The Anatomy of a 12-mm. Pig 
anlage empties into the bile duct close to the intestine. The dorsal 
and larger portion, separated from the ventral by the portal vein, opens 
into the duodenum below the bile duct. The adult pancreas is formed 
of a band of gland-tissue extending from the bilary duct to the duct of 
Santorini, which opens into the intestine 15 cms. lower down. From 
this band extend two tails which seem to correspond with the two anlages. 
The ventral duct, that of Wirsung, is represented by impervious fibrous 
tissue. The duct of Santorini, from the dorsal anlage, is the only one 
retained in the adult pig. 
The intestine extends into the umbilical cord as a simple loop. 
After receiving the yolk stalk which is now very slender and scarcely 
pervious, it returns to the body. A dilatation marks the position of the 
coecum, which in the adult is a voluminous pouch without an appendix. 
The rectum, just before entering the cloaca, is nearly occluded by an 
epithelial proliferation which separates the intestinal and urinary 
tracts. In a 20-mm. pig the anus is distinct from the urogenital sinus, 
and the proliferation has wholly disappeared. TI have found a similar 
plug in rabbits, and Keibel, 96, has figured one in an 11.5-mm. human 
embryo. He describes it as an “ accidental and insignificant adhesion.” 
Gasser discovered a corresponding structure in the chick which has 
been fully described and its function explained by Minot, oo, 2. 
The urogenital system consists of very large Wolffian bodies outlined 
in Plate IV, and of the genital ridges. The kidneys are represented by 
their pelves, Plate III. From the latter, the slender ureters pass to the 
Wolffian ducts, which in turn unite with the allantois and enter the 
cloaca. 
The cloaca is closed by its “ membrane,” or rather by the approxi- 
mation of its borders which form a raphe. This is to open at its ends, 
giving rise to the anal and urogenital apertures, and to remain fused 
between them, forming the perineal raphe. 
Heart, Arteries, and Veins.—The heart, because of its trabecular 
structure, must be drawn diagrammatically. In Plate IIL it is shown cut 
through on the left side of the median septa. The constriction be- 
tween the auricles forms a partial septum. Below this and toward the 
left side of the embryo is the large foramen ovale, bounded partly by 
the auricular wall and partly by the thin wavy septum superius. This 
septum is attached ventrally to a thick mesenchymal mass, from which 
*In man, His (85, pp. 19 and 24) has figured the dorsal anlage in its later stages as 
emptying below the bile duct, as in the pig. Hamburger, Schirmer and others find 
the duct of Santorini opening above the bile duct. 
