VENTRAL DISSECTIONS 143 



is now divided by the palatine folds into the upper nasal passages and lower 

 oral cavity. In the lateral walls of the nasal passages develop the anlages of the 

 turbinate bones. On the floor of the mouth z nd pharynx, the tongue and epiglottis 

 become more prominent. The trachea and esophagus elongate and the lungs lie 

 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 Hmb 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 cacum, 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 median surfaces of the mesonephroi. The metanephroi 

 have increased rapidly in size and have shifted cephalad. Proximal to the allan- 

 toic 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 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. 

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



The heart, as may be seen by comparing Figs. 96 and 147, 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, 

 profound changes in the positions of the vessels are thus brought about, for the 

 vessels must shift their positions with the organs which they supply. 



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 section, 

 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 



