DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 43 



of the embryo, so that it is not shown in the figure. The dim out- 

 line of the now convoluted heart may be seen if the "cleared" 

 embryo be viewed by transmitted light ; it is not shown in the 

 figure. The allantois (al) is a rounded sac of considerable size just 

 anterior to the' tail. Four gill clefts (g 1 * 4 ) are now present; the 

 most posterior one is more faint than is represented in the figure, 

 and it could not be definitely determined from a surface view 

 whether or not it opened to the exterior. The mandibular fold 

 ( md) is now fairly well outlined, but there is as yet no sign of the 

 maxillary process. 



Figure 16a is the most anterior of a series of transverse sections 

 made of an embryo of the approximate age of the surface view just 

 described; it passes through the tip of the forebrain (fb) and shows 

 the nasal pit ( // ) of the right side. The great thickening of ecto- 

 derm in the region of the nasal invagination is represented by a 

 solid line. Owing to the obliquity of the section, the left nasal pit 

 was not cut. The mesoblast is quite dense and contains two or three 

 small blood-vessels near the roof of the brain. The plane of this 

 section, owing to the cranial and body flexure, cut the embryo also 

 in the region of the pharynx ; this part of the section was, as a 

 matter of convenience, omitted from the drawing. 



Figure 16b is in reality more anterior in position considering the 

 entire embryo, than the preceding; but the region of the embryo 

 represented is most posterior, so that it is described at this point. 

 The greatly elongated outline of the brain is due to its being cut 

 through the region of flexure, so that the forebrain ( fb) or, per- 

 haps, midbrain, is shown at one end and the hindbrain (hb) at the 

 other. The walls of these cavities are somewhat wrinkled and irreg- 

 ular and their constituent cells are beginning to show slight differ- 

 entiation, though this is not shown in the figure. On the left side 

 are seen a couple of darkly stained masses ; one is the origin of a 

 cranial nerve (en) ; and the other is one of the auditory vesicles 

 (o), which is still more irregular in outline than it was in the pre- 

 ceding stage. The only blood-vessels to be seen are a few very 

 small ones that lie close to the wall of the brain. The ectoderm is 

 quite thin at all points. 



Figure i6r, the largest section of this series, passes through the 

 forebrain in the region of the eyes and through the gill clefts. The 

 forebrain (fb) exhibits on the left a marked thickening of its wall 

 (ch ), the edge of the cerebral hemisphere of that side, which is just 

 beginning to develop ; on its right side the lower part of the fore- 

 brain is connected by a well-marked optic stalk ( os) with the optic 



