230 STUDY OF PIG EMBRYOS. 



on its own axis that its dorsal border nearly faces the left. Just below the en- 

 trance of the oesophagus, the cardiac end of the stomach has a small diverticulum, 

 which is characteristic for the pig, but does not occur in man. Below the stom- 

 ach the digestive canal is narrow and forms a long loop, II, which extends t6 the 

 umbilical opening, where it joins the neck of the yolk-sac, Yk. This portion of 

 the tract gives rise to the duodenum, jejunum, and most of the ileum. Beyond 

 the yolk-sac . the intestinal tract is continued as a narrow tube, Col, which leads to 

 a considerable expansion, the cloaca, Clo, at the base of the tail. From the cloaca 

 there -extends into the tail a very narrow prolongation of the entodermal canal 

 known as the tail-gut, or caudal intestine, In.caud. Into the cloaca also open the 

 Wolffian ducts, W.D, which are the .ducts of the primitive excretory organs of the 

 embryo. The duct on the left side is represented as cut off close to the cloaca. 

 More of the right duct is included in order to show the anlage of the true or per- 

 manent kidney, Ki, which is budding off from it. Returning to the portion of the 

 intestine (duodenum) next to the stomach, we find clearly displayed the anlage of 

 the dorsal pancreas, Pan.d. The anlage of the ventral pancreas also appears, Pan.v, 

 but less clearly. It is an elongated mass of entoderm, lying to the right of the 

 duodenum and ventral to the portal vein, V.port. It takes its origin from the duct 

 of the liver. The liver itself, Li, has already acquired a considerable size. On 

 its ventral surface lies the gall-bladder, which is connected with the liver substance 

 by .several cords of hepatic cells. In the adult, only one connection between the 

 gall-bladder and the liver persists. 



The central nervous system is represented as* seen in median section, display- 

 ing the cavity and the inner surfaces of the walls of the cavity. In the region of 

 the head the tube is already much dilated to make the brain. In the region of 

 the body it narrows to form the spinal cord, Sp.c, which gradually diminishes in 

 diameter toward the tail. The brain is clearly subdivided into its three primary 

 vesicles: the fore-brain, F.B; the mid-brain M.B; and the hind-brain, H.B. Owing 

 to the head-bend, the mid-brain forms an arch. It is less in diameter than either 

 mid-brain or hind-brain. The demarcation between the mid-brain and hind-brain, 

 known as the isthmus, Isth, is strongly marked. The hind-brain, H.B, is longer 

 than the mid- and fore-brain combined. It diminishes in diameter posteriorly, and 

 passes without definite demarcation into the spinal cord. Owing to the head-bend, 

 the fore-brain, F.B, is brought to underlie, as it were, the anterior portion of the 

 hind-brain, and to overlie the heart. The heart is a very large organ, which is 

 represented in the figure somewhat to the left of the median line. It consists of a 

 smaller dorsal or upper chamber the atrium or auricle, and a lower larger cham- 

 ber the ventricle. The auricle shows two openings by which it communicates with 

 the right auricle. The upper of these, F.ov, is the foramen ovale; the lower is 

 the so-called interatrial foramen, For.ia. The small pulmonary vein, V.pul, opens 

 directly into the auricle. The ventricle has a trabecular or sinusoidal structure. 

 The cavity in the drawing is that of the left side. It communicates with the cavity 



