THE ANATOMY OF A SIX MM. PIG EMBRYO 



97 



R. alrium 



R. ventricle- 



'Bulbils cordis 

 L. atrium 



—"L. ventricle 



The cloaca, a dorso-ventrally expanded portion of the hind-gut, gives off 

 cephalad and ventrad the allantoic stalk. This is at first a narrow tube, but soon 

 expands into a vesicle of -large size, a portion of which is seen in Fig. 95. Dorso- 

 laterad the cloaca receives the primary excretory (mesonephric) ducts. The hind- 

 gut is continued into the tail as the tail gut (postanal gut) which dilates at its ex- 

 tremity as in the 7.8 mm. pig described by Thyng. Th^jnid-ventral wall pfthe 

 cloaca is fused to the adjacent ectoderm to form the cloacal membrane. In this 

 region later the anus arises (Fig. 105). The postanal gut soon disappears. 



Urogenital System. — This consists of the mesonephroi, the mesonephric (Wolf- 

 fian) ducts, the anlages of the metanephroi, the cloaca, and the allantois. The form 

 of the mesonephroi is seen in Figs. 95 and 97. Each consists of large vasculat glom- 

 eruh associated with coiled tubules Hned with cuboidal epithelium and opening 

 into the mesonephric duct (Figs. 114 and 208). The Wolffian ducts, beginning 

 at the anterior end of the mesonephros, 

 curve at first along its ventral, then along 

 its lateral surface. At its caudal end 

 each duct bends ventrad and to the mid- 

 line where it opens into a lateral expansion 

 of the cloaca (Fig. 96). Before this junc- 

 tion takes place, an evagination into the 

 mesench)ane from the dorsal wall of each 

 mesonephric duct gives rise to the anlages 

 of the metanephroi, or permanent kid- 

 neys. A slight thickening of the mesothelium along the median and ventral sur- 

 face of each mesonephros forms a Hght-colored area, the genital fold (Fig. 96). 

 This area is pointed at either end and confined to the middle third of the kidney. 

 It is the anlage of the genital gland from which either testis or ovary is developed. 



Blood Vascular System. — The heart Ues in the pericardial cavity, as seen in 

 Fig. 96. The atrial region (Fig. 99), as in the 4.2 mm. human embryo, has given 

 rise to two lateral sacs, the right and left atria. The bulbo-ventricular loop has 

 become differentiated into right and left ventricles much thicker walled than 

 the atria. The right ventricle is the smaller and from it the bulbus passes be- 

 tween the atria and is continued as the ventral aorta. Viewed from the caudal 

 and dorsal aspect (Fig. 100) the sinus venosus is seen dorsal to the atria. It opens 

 into the right atrium and receives from the right side the right common cardinal 

 vein, from the left side the left common cardinal. These veins drain the blood 

 from the body of the embryo. Caudally the sinus venosus receives the two 

 7 



Fig. 99. — Ventral and cranial surface of the 

 heart from a 6 mm. pig embryo. X 14. 



