Fig. ioi. — Pig Embryo op 12.0 mm. Reconstruction from the Transverse Sections, Series 5. 

 For the most part the organs represented are in or near the median plane. The drawing illustrates especially 

 the disposition of the alimentary tract, the arterial system, and the heart. A.bas, Basilar artery, formed by 

 the union of the two vertebral arteries, A. v, and joined, under the mid-brain, by the anterior ends of the 

 internal carotids. A. cm. Caudal artery, the small median prolongation of the dorsal aorta. All, The 

 allantois ; it is joined by the Wolffian duct, and empties into the cloaca, an.pl, The cloacal mem- 

 brane. Ao, Median dorsal aorta. A0.1t, Right descending aorta; a small vessel connecting the dorsal 

 ends of the third (carotid) and fourth aortic arches. Ao.D, Main right descending aorta, passing down- 

 ward to join its fellow and form the median aorta. A.s, Subclavian artery. An, Left auricle of the heart. 

 A.um, The umbilical artery, which runs in the mesodermic wall of the allantois and joins the caudal end of 

 the dorsal aorta. A.v, Vertebral artery. A.vi, The vitelline artery, which becomes the superior mesenteric 

 artery of the adult, c. Artery known as the cceliac axis in the adult, car.e, External carotid artery, arising 

 from the base of the third aortic arch, car.i, Internal carotid artery. Cbl, Cerebellum. d, Left duct of 

 Cuvier. Dien, Diencephalon. D. V, Ductus venosus Arantii. ep, Small plug of entodermal epithelium in 

 the rectum ; there are several small irregular passages through the plug. Epgl, Anlage of the epiglottis. 

 f, Interventricular foramen, opening into the space a, b, r, of Fig. 105. F.M, Foramen of Monro, f.o, 

 Foramen ovale, between the cardiac auricles, g, Gall-bladder. In, Entodermal wall of the intestine, is, 

 One of the series of intersegmental arteries. A'i, Enlarged blind end of ihe ureter, the anlage of the renal 

 pelvis. Lar, Anlage of the larynx, consisting chiefly of an epithelial plate. Li, Liver. Lu, Entodermal 

 portion of the lung. M.b, Mid-brain. Md.ob, Medulla oblongata. If, A spinal nerve. Neu, Neuromeres 

 of the hind-brain. Oe, CEsophagus. op, Stalk of the optic vesicle ; the stalk is the anlage of the optic 

 nerve. P. A, Pulmonary artery, pan. Dorsal anlage of the pancreas, the narrow part of which becomes 

 the duct ; behind the duct is the anlage of the ventral pancreas, which arises as an outgrowth from the 

 duct of the gall-bladder, p.c. Pericardial cavity. P.V, Pulmonary vein. Sp. c, Spinal cord. St, Stomach ; 

 the letters are placed beside the diverticulum which is characteristic of the pig and is not found in man. 

 /, Posterior portion of the tongue, an eminence on the floor of the pharynx, fcrmed by the union of the 

 ventral ends of the third and fourlh gill arches. T.i, Tuberculum impar, the anlage of the anterior portion 

 of the tongue. Tra, Entodermal trachea. Ug.si, Cloaca. Um.d, Right umbilical vein. Ven, The left 

 ventricle, the letters being placed on the septum inferius. V.mes, Superior mesenteric vein. V.port, 

 Portal vein. V.ri, Vitelline vein. IV. D, Wolffian duct, x, Anastomosis between thvi right and left 

 cardinal venous systems. )'/', Volk-sac. Yk.s, Entodermal yolk-stalk, connecting the yolk-sac with t lie 

 intestine. X '4 diams. — {Drawn by Dr. /•'. 7'. Lewis.) 



