DEVELOPMENT OF THE AMERICAN AEEIGATOR — REESE 65 



almost the same age as the preceding, to show the further de- 

 velopment of the medullary folds. After Clarke. 



Figure; 6. Dorsal view of an embrye only a day or two older than the pre- 

 ceding. After Clarke. 



Figi RES 6a 6i A series nf transverse sections of this stage. X 43. 



FIGURES ~ja-~h. A series of transverse sections of an embryo slightly older 

 than the one shown in figures 4-6. X 43. (No surface view of 

 this stage is figured. ) 

 8 and 8a. Dorsal and ventral views respectively of an embryo with 

 fiye pairs of mesoblastic somites. X 20. (Drawn by trans- 

 mitted light.) 

 8b and 8c". Two sagittal sections of an embryo of this stage. X 43. 



FIGURES 8d-8/. A series of transverse sections of the embryo represented in 

 figures 8 and 80. X 43. 

 Qa-gm. A series of transverse sections of an embryo somewhat more 

 advanced in development than the one represented in the last 

 series. X 43. 



Figures 10 and 101;. Dorsal and ventral views respectively of an embryo with 

 eight pairs of mesoblastic somites. X 20. (Drawn chiefly by 

 transmitted light. ) 



Figure II. Dorsal view of an embryo with fourteen pairs of mesoblastic 

 somites. The area pellucida and the developing vascular area 

 are shown, the latter having a mottled appearance. The pushing 

 of the head under the blastoderm is also shown. X 20. (Drawn 

 chiefly by transmitted light.) 



Figures lia-llk. A series of transverse sections of an embryo of this stage. 

 X43. 



Figure 12. Dorsal view of an embryo with about seventeen pairs of meso- 

 blastic somites. Part of the area pellucida is represented. 

 ( Both transmitted and reflected light were used in making the 

 drawing.) X 13. 



Figures \2a-12g. A series of transverse sections of an embryo of this stage. 



X43- 



FlGURE i.v Surface view of an embryo with about twenty pairs of meso- 

 blastic somites. X (about) 15. (Drawn with both reflected 

 and transmitted light.) 



Figures 13-13/. A series of transverse sections of an embryo slightly more 

 developed than the one shown in figure 13. X 20. 



Figure I3g. A sagittal section of an embryo <>f about the age of the one 

 represented in figure 13. X 20. 



14. Head of an embryo with one pair of gill clefts; ventro-lateral 



view. X 13. 



15. Profile view of the head of an embryo with three pairs of gill 



clefts. X 13. 

 Figures \$a-\$c. A series of transverse sections of an embryo of about the 



age of the one represented in figure 15. X 20. 

 Figure 15/. A horizontal section through the anterior region of an embryo of 



the age of that shown in figure 15. X 20. 



16. Surface view in profile of an embryo with four pairs of gill clefts. 



X (about) 12. 



5— AL 



