The Development of the Alhgator 293 



STAGE XI 



Figure 14 (Plate XXI.) 



Only the anterior region of this embryo is shown 

 in the figure, which is a ventro-lateral view. While 

 there is some change in the general shape and in 

 parts of the head, the reason for figuring this 

 stage is to show the first gill cleft (g'), which lies 

 at an acute angle to the long axis of the neck behind 

 the eye (e). The cleft is narrow but sharp and 

 distinct in outline; it shows neither in this nor in 

 the following stages the branched, Y-shaped out- 

 line mentioned by Clarke. 



STAGE XII 



Figures 15-15/ (Plates XXI., XXII.) 



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'~^) 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 

 posterior, as has been described by Clarke, from 

 which, or in connection with which according to 

 Clarke, the fourth cleft will develop. All three 

 clefts may be distinctly seen to open entirely 



