The Development of the AlHgator 331 



gated cavity whose ventral wall is in close contact 

 with a group of small, darkly staining alveoli (p), 

 the pituitary body. Extending posteriorly from 

 the pituitary body is a gradually thickening mass of 

 cartilage (hp), which surrounds the anterior end 

 of the notochord {nt) and may be called the basilar 

 plate. In its anterior region, where the section is 

 nearly median, the spinal column shows its canal, 

 with the enclosed spinal cord, while toward the 

 posterior end of the figure the vertebrae are cut to 

 one side of the middle line, and hence show the 

 neural arches {na) with the alternating spinal 

 ganglia {sg). Near the posterior end of the figure 

 the pelvic girdle {pi) is seen. The largest organ of 

 the embryo, as seen in this section, is the heart, of 

 which the ventricle (im) seems to be closely sur- 

 rounded, both in front and behind, by the auricles 

 {au). The liver Qi) is the large, reticular mass 

 back of the heart. Dorsal and anterior to the 

 liver is the lung {lu), now of considerable size and 

 development. The enteron is cut in several places 

 {oe, i) and its walls are beginning to show some 

 differentiation, though this cannot be seen under 

 the magnification here used. One of the Wolffian 

 bodies is seen as a huge mass of tubules {wt) extend- 

 ing from the pelvic region, where the mass is 

 greatest, to the region of the lungs. The Wolf- 

 fiian tubules stain darkly and the whole structure 

 forms a very striking feature of the section. Dor- 

 sal to the posterior end of the Wolffian body is a 



