THE ANATOMY OF A SIX MM. PIG EMBRYO 



99 



R. atrium 



by Thyng; it soon disappears. The mid-ventral wall of the cloaca is 

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

 region later the anus arises. 



Urogenital System. This consists of the mesonephroi, the mesonephric 

 (Wolffiari) ducts, the anlages of the metanephroi, the cloaca, and the allan- 

 tois. The form of the mesonephroi is seen in Figs. 95 and 97. Each 

 consists of large, vascular glomeruli, 

 associated with coiled tubules lined 

 with cuboidal epithelium and opening 

 into the mesonephric duct (Figs. 114 

 and 208). The Wolffian ducts, begin- R venir - lc i e . 

 ning at the anterior end of the meso- 

 nephros, curve at first along its ventral, 

 then along its lateral surface. At its 

 caudal end each duct bends ventrad and 

 to the mid-plane where it opens into a 

 late'ral expansion of the cloaca (Fig. 96). 

 Before this junction takes place, an evagination into the mesenchyme 

 from the dorsal wall of each mesonephric duct gives rise to the anlages of 

 the metanephroi, or permanent kidneys. A slight thickening of the 

 mesothelium along the median and ventral surface of each mesonephros 

 forms a light-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. 



-Bulbus cordis 

 R. atrium 



Jj L. ventricle 



FIG. 99. Ventral and cranial surface 

 of ,the heart from a 6 mm. pig embryo. 

 X 14- 



L. common 



cardinal vein 



L. vitelline vein 



R. vitelline vein 



Left ventricle 



TJR. ventricle 



FIG. 100. Dorsal and caudal view of the heart from a 6 mm. pig embryo. X 21. 



Blood Vascular System. The heart lies in the pericardial cavity, as 

 seen in Fig. 96. The atrial region (Fig. 99), as in the 4.2 mm. human em- 

 bryo, 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 between the atria and is continued as the 



