DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 59 



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 "t>f 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 (vn) seems to be closely surrounded, both in 

 front and behind, by the auricles (au). The liver (li) 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 (oc, 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 (zvt) extending from the pelvic region, where 

 the mass is greatest, to the region of the lungs. The Wolffian 

 tubules stain darkly and the whole structure forms a very striking 

 feature of the section. Dorsal to the posterior end of the Wolffian 

 body is a small, oval mass of very fine tubules (k), which do not 

 stain so darkly as do the Wolffian tubules ; this mass is apparently 

 the beginning of the permanent kidney, the metanephros. Its 

 tubules, though their origin has not been determined, seem to be 

 entirely distinct from the tubules of the Wolffian body. 



A single vertical section through the anterior part of the head of 

 an embryo of this age has been represented in figure 236. On the 

 right side the plane of the section cut through the lens of the eye 

 (In) ; on the left side the section was anterior to the lens. The 

 upper (nl) and lower (//) eyelids are more evident here than in the 

 surface view. Owing to the hardness of the lens, its supporting 

 structures were torn away in sectioning. The vitreous humor is not 

 represented in the figure. The superior (ar) and inferior (Ir) recti 

 muscles are well shown on the right side; they are attached to the 

 median part of a Y-shaped mass of cartilage (se), which may be 

 termed the sphenethmoidal cartilage. Between the branches of this 

 Y-shaped cartilage the anterior ends of the cerebral hemispheres 

 (ch) — better called, perhaps, the olfactory lobes — are seen. Be- 

 tween the lower end of the sphenethmoidal cartilage and a dorsally 

 evaginated part of the pharynx are two small openings (pn) ; when 

 traced forward these tubes are found to open into the convoluted 

 nasal chamber, while a short distance posterior to the plane of this 

 figure they unite with each other and open almost immediately into 

 the pharynx. The rather complicated structures of the nasal pas- 

 sages of the alligator have been described by the writer in another 

 paper (12). In the lower jaw the cartilage (mk) is seen on either 



