VENTRAL DISSECTIONS 



loop is not coiled, but its diverticulum, the exeunt, is more marked. Caudally, the rec- 

 tum, or straight gut, has completely separated from the urogenital sinus and opens to the 

 exterior through the anus. 



Of the urogenital organs, the genital folds have become the prominent genital glands, 

 attached to the median surfaces of the mesonephroi. The metanephroi have increased 

 rapidly in size and have shifted cephalad. Proximal to the allantoic stalk the adjacent 

 portion of the urogenital sinus has dilated to form the bladder. As the urogenital sinus 

 grows, it takes up into its wall the proximal ends of the mesonephric ducts, so that these and 

 the ureters have separate openings into the sinus. Owing to the unequal growth of the 

 sinus wall, the ureters open near the base of the bladder, the mesonephric ducts more cau- 

 dally into the urethra. The phallus now forms the penis of the male or the clitoris of the 

 female. Cranial to the metanephros, a new organ, the suprarenal gland, has developed. 

 This is a ductless gland and is much larger in human embryos. 



The heart, as may be seen by comparing Figs. 96 and 147, athough at first pressed 

 against the tip of the head, shifts caudally, until in the 35 mm. embryo, it lies in the thorax 

 opposite the first five thoracic nerves. Later it shifts even further caudad. The same 

 is true of the other internal organs, the metanephros excepted. As the chief blood vessels 

 are connected with the heart and viscera, profound changes in the positions of the vessels 

 are thus brought about, for the vessels must shift positions with the organs which 

 they supply. 



~ Trachea 



Septum transversum 

 Stomach 



Mesonephric duct 

 Mesonephros 



Umbilical artery 

 Rectum 



Lung 



Midlerian duct anlage 



Ctecum 



Small intestine 

 Large intestine 



Allantoii 



Codom 



FIG. 148. Ventral dissection of a 15 mm. pig embryo, showing lungs, digestive canal and 

 mesonephroi. The ventral body wall, heart and liver have been removed and the limb buds 

 cut across. X 6. 



Ventral Dissections. Ventral dissections of the viscera are very easily made. With 

 the safety razor blade, start a cut in a coronal plane through the caudal end of the embryo 

 and the lower limb buds (Fig. 148). Extend this cut laterad and cephalad through the 

 body wall and the upper limb bud. The head may be cut away in the same plane of sec- 

 tion, and the cut continued through the body wall and upper limb bud of the opposite side 

 back caudally to the starting point. Section the embryo in a coronal plane, parallel with 

 the first section and near the back, so that the embryo will rest upon the flattened surface. 

 With forceps, now remove the ventral body wall. By tearing open the wall of the umbilical 

 cord along one side, it may be removed, leaving the intestinal loop intact. Pull away the 

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