EXPLANATION OF FIGURES 1-26 ON PLATES VI.-XXVIII.

All of the figures, with the exception of the photographs and those copied by permission from S. F. Clarke, were drawn under a camera lucida.

The magnification of each figure, except those from Clarke, is indicated below.

The photographs were made by the author, and were enlarged for reproduction by the photographic department of the Smithsonian Institution. The other surface views were made, under the author’s direction, by Miss C. M. Reese.

With the exception of Stage III., all of the figures of any one stage are given the same number, followed where necessary by a distinguishing letter, so that it is possible to tell at a glance which section and surface views belong together. The transverse sections are all cut in series from anterior to posterior.

[Figures 1.] Surface view of egg. × ²⁄₃.
[Figures 1a.] Egg with part of the shell removed to show the chalky band in the shell membrane. × ²⁄₃.
[Figures 2 and 2a]. Dorsal and ventral views respectively of the blastoderm before the formation of the notochord, medullary folds, etc. After Clarke.
[Figures 2b-2f]. Transverse sections of an embryo of the age represented in Figures 2 and 2a. × 43.
[Figures 3] and [3a]. Ventral and dorsal views respectively of an embryo a few days older than that represented in Figures 2 and 2a. After Clarke.
[Figures 3b]-[3m]. Transverse sections of an embryo of the age shown in Figures 3 and 3a. × 43.
[Figures 3n and 3o]. Two sagittal sections of an embryo of the same stage as Figures 3 and 3a. × 43.
[Figures 4 and 4a]. Dorsal and ventral views respectively of a slightly older embryo than the one shown in Figures 3 and 3a. Figure 4a shows only the head region. After Clarke.
[Figures 5 and 5a]. Dorsal and ventral views respectively of an embryo of almost the same age as the preceding, to show the further development of the medullary folds. After Clarke.
[Figures 6]. Dorsal view of an embryo only a day or two older than the preceding. After Clarke.
[Figures 6a-6i]. A series of transverse sections of this stage. × 43.
[Figures 7a]-[7h]. A series of transverse sections of an embryo slightly older than the one shown in Figures 4-6. × 43. (No surface view of this stage is figured.)
[Figures 8 and 8a]. Dorsal and ventral views respectively of an embryo with five pairs of mesoblastic somites. × 20. (Drawn by transmitted light.)
[Figures 8b and 8c]. Two sagittal sections of an embryo of this stage. × 43.
[Figures 8d]-[8j]. A series of transverse sections of the embryo represented in Figures 8 and 8a. × 43.
[Figures 9a]-[9m]. A series of transverse sections of an embryo somewhat more advanced in development than the one represented in the last series. × 43.
[Figures 10] and [10a]. Dorsal and ventral views respectively of an embryo with eight pairs of mesoblastic somites. × 20. (Drawn chiefly by transmitted light.)
[Figures 11]. 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. × 20. (Drawn chiefly by transmitted light.)
[Figures 11a]-[11k]. A series of transverse sections of an embryo of this stage. × 43.
[Figures 12]. Dorsal view of an embryo with about seventeen pairs of mesoblastic somites. Part of the area pellucida is represented. (Both transmitted and reflected light were used in making the drawing.) × 13.
[Figures 12a]-[12g]. A series of transverse sections of an embryo of this stage. × 43.
[Figures 13]. Surface view of an embryo with about twenty pairs of mesoblastic somites. × (about) 15. (Drawn with both reflected and transmitted light.)
[Figures 13a]-[13f]. A series of transverse sections of an embryo slightly more developed than the one shown in Figure 13. × 20.
[Figures 13g]. A sagittal section of an embryo of about the age of the one represented in Figure 13. × 20.
[Figures 14]. Head of an embryo with one pair of gill clefts; ventro-lateral view. × 13.
[Figures 15]. Profile view of the head of an embryo with three pairs of gill clefts. × 13.
[Figures 15a]-[15e]. A series of transverse sections of an embryo of about the age of the one represented in Figure 15. × 20.
[Figures 15f]. A horizontal section through the anterior region of an embryo of the age of that shown in Figure 15. × 20.
[Figures 16]. Surface view in profile of an embryo with four pairs of gill clefts. × (about) 12.
[Figures 16a]-[16f]. A series of transverse sections of an embryo of the approximate age of the one represented in Figure 16. × 20.
[Figures 16g]. A sagittal section of an embryo of the age (possibly slightly younger) of the one represented in Figure 16. × 20.
[Figures 17]. Surface view in profile of an embryo at the time of origin of the limbs. × (about) 5.
[Figures 17a-17g]. A series of transverse sections of an embryo of the age of the one represented in Figure 17. × 7.
[Figures 18]. Surface view in profile of the head of an embryo slightly larger than, though of about the same state of development as, the one represented in Figure 17. Reproduced here chiefly to show the gill clefts. × (about) 3.
[Figures 19]. Surface view of an embryo somewhat more developed than the one just described. × (about) 3.
[Figures 20]. Surface view of an embryo older than the one represented in Figure 19; with well-developed manus and pes. × (about) 5.
[Figures 20a]-[20j]. A series of transverse sections of an embryo of the age of the one represented in Figure 20. × 7.
[Figures 21]. A photograph of a living embryo in the egg, showing the allantois, yolk mass, etc. The embryo is somewhat more developed than the one shown in Figure 20. × ²⁄₃.
[Figures 22]. A photograph of a still larger embryo, removed from the shell and freed from the fetal membranes. × (about) 1.
[Figures 23]. A photograph of a still more advanced embryo, in which the digits are quite evident and the scales are beginning to show. × (about) 1.
[Figures 23a]. A sagittal section of an embryo of the age of the one represented in Figure 23; the tail has not been shown in this figure. × (about) 3.
[Figures 23b]. A vertical section through the head of an embryo of about the size (perhaps slightly smaller) of the one shown in Figure 23. × (about) 3.
[Figures 24]. A photograph of an older embryo in which the pigmentation of the scales is evident, though not shown in the figure. × (about) 1.
[Figures 25]. A photograph of an embryo in which the pigmentation and the development of the body form are practically complete. The allantois, unabsorbed yolk, etc., have been removed. × (about) ³⁄₄.
[Figures 26]. A photograph of a just-hatched alligator, of an alligator egg, and of a young alligator in the egg just before hatching. × (about) ³⁄₇.