LATERAL DISSECTION OF THE VISCERA 



105 



R.atriun 



R.Ventrlcle 



L.ventricle 



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. 90. Dorso- 

 laterad the cloaca receives the primary excretory (Wolffian ducts}. The hind-gut 

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

 tremity as in the 7.8 mm. pig described by Thyng. 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 (Fig. 100). The post-anal gut soon disap- 

 pears. 



The urogenital organs consist of the mesonephroi, the mesonephric ducts, 

 the anlages of the metanephroi, the cloaca and the allantois. The form of 

 the mesonephroi is seen in Figs. 90 and 92. Each consists of large vascular glom- 

 eruli associated with coiled tubules lined with cuboidal epithelium and opening 

 into the mesonephric duct (Figs. 107 and 109). The Wolfnan 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 midline, where it opens 

 into a lateral expansion of the cloaca. Be- 

 fore 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 meso- 



thelium along the median and ventral surface of each mesonephros forms a 

 light-colored area, the genital fold (Fig. 91). 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 lies in the pericardial cavity as seen in 

 Fig. 91. The atrial region (Fig. 94), 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 

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

 and dorsal aspect (Fig. 95), 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 



FIG. 94. Ventral and cranial sur- 

 face of the heart from a 6 mm. pig 

 embryo. X 14. 



