DEVELOPMENT OF Till-. AMERICAN ALLIGATOR — REESE 39 



but sharp and distinct in outline; it shows, neither in this nor in 

 che following stages, the branched. Y-shaped outline mentioned by 

 Clarke. 



Stage XII 

 Figures 15-15/" ( Plates XVI, XVII) 



In this stage, also, only the anterior region of the embryo is 

 figured in surface view. The shape of the head is about the same 

 as in the preceding stage, but it is drawn in exact profile. Three 

 gill clefts (g 1 ' 3 ) are now present, and are wide and distinct. The 

 first cleft, as in the preceding stage, lies at an acute angle to the 

 long axis of the pharynx and nearly at right angles to the second 

 cleft. The third cleft sends a wide branch (g*) toward the pos- 

 terior, as has been described by Clarke, from which, or in connec- 

 tion with which according to Clarke, the fourth cleft will develop. 

 All three clefts may be distinctly seen to open entirely through the 

 pharyngeal wall. The outlines of the visceral folds, especially of 

 the mandibular, begin to be apparent. The nasal pit (n) now 

 shows as a round depression in front of the more definitely outlined 

 eye (e). The auditory vesicle (0) is so deep beneath the surface 

 that it may be seen only by transmitted light. 



Figures 15a-? represent transverse sections of an embryo of 

 about this general state of development, except that the gill clefts 

 are not so definitely open as in the surface view. 



Figure 15a, the most anterior section of the series, passes through 

 the forebrain (fb) in the region of the eyes, and through the hind- 

 brain {lib) anterior to the auditory vesicles. The forebrain is here 

 a large cavity with a dense wall of a comparatively even thickness. 

 Owing probably to the section not being exactly in the transverse 

 plane, the eyes are cut in different regions, that on the left (ov) 

 being cut through its stalk, while that on the right (oc) is cut near 

 its middle region and hence does not show any connection with the 

 forebrain. The almost complete obliteration of the cavity of the 

 optic vesicle to form the optic cup by the invagination of the outer 

 wall of the vesicle is shown on the right side of the section (oc). 

 The lens vesicle (Iv) is completely cut off from the superficial ecto- 

 derm (cc), which is comparatively thin. The hindbrain (lib) has 

 the usual shape for that structure. Its ventral wall is dense and 

 thick, while its roof is reduced to the usual thin, wrinkled mem- 

 brane. Close to the floor of the hindbrain lies the notochord (lit), 

 which is large and is distinctly vacuolated. To the right of the 

 hindbrain a large mass of darkly stained cells (en) is one of the 



