152 



THE DISSECTION OF PIG EMBRYOS FOR STUDY 



Trachea 



Septum 

 ' transv. 



more and more caudad. The dorsal portion of the septum transversum, the 

 anlage of a portion of the diaphragm, is thus carried caudad and although origi- 

 nally, when traced from the dorsal body wall, it was directed caudad and ventrad 

 now it curves cephalad and ventrad, bulging cephalad into the thorax. The 

 proximal limb of the intestinal loop elongates rapidly and, beginning with the 

 duodenum, becomes flexed and coiled in a characteristic manner. The distal 

 limb of the intestinal loop is not coiled, but its diverticulum, the ccecum, is more 

 marked. Caudally the rectum, 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 med- 

 ian surfaces of the meso- 

 nephroi. The metanephroi 

 have increased rapidly in 

 size and have shifted ceph- 

 alad. The proximal portion 

 of the allantoic stalk has di- 

 lated and, with the adjacent 

 part of the urogenital sinus, 

 forms the bladder. As the 

 urogenital sinus grows it 

 takes up into its wall the 

 proximal ends of the meso- 

 nephric ducts, so that these 

 and the ureters have sepa- 

 rate openings into the sinus. 

 Owing to the unequal growth 



of the sinus wall, the ureters cpen near the base of the bladder, the mesonephric 

 ducts more caudally 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. These are ductless glands and are much 

 larger in human embryos. 



The heart, as may be seen by comparing Figs. 91 and 142, although 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, 



Large 

 intestine 



/Illanto, 



CoeLom 



FIG. 143. Ventral dissection of a 15 mm. embryo, show- 

 ing lungs, digestive canal and mesonephroi. The ventral body 

 wall, heart and liver have been removed and the limb buds 

 cut across. X 6. 



