64 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



sg, spinal ganglion. u, umbilical stalk. 



sm, splanchnic mesoblast. it!, upper lid of eye. 



sn, spinal nerve. '"'. superior rectus muscle of eye. 



so, somatic mesoblast. v'"-"', first, second, and third cere- 



st, stomodaeum. bral vesicles. 



sy, sympathetic nervous system. va, vascular area. 



/, tail. vm, vitelline membrane. 



ta, trachea. vn, ventricle of heart. 



tg, thyroid gland. w, vitelline blood-vessels. 



//;. thickening and posterior limit of sf. i^d. Wolffian duct. 



tn, tongue. wdo, opening of Wolffian duct. 



to, tooth anlage. wr, Wolffian ridge. 



tr, truncus arteriosus. wt, Wolffian tubules. 



tv, third ventricle of brain. y. yolk. 



tv' , third ventricle of brain. 



EXPLANATION OF FIGURES 1-26 ON PLATES I-XXIII 



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 repro- 

 duction 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. 



Figure i. Surface view of egg. X 2/3. 



id. Egg with part of the shell removed to show the chalky band in 

 the shell membrane. X 2/3. 

 Figures 2 and 2a. Dorsal and ventral views respectively of the blastoderm be- 

 fore the formation of the notochord, medullary folds, etc. After 

 Clarke. 

 2b-2f. Transverse sections of an embryo of the age represented in 

 figures 2 and 2a. X 43. 



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. 

 30-3111. Transverse sections of an embryo of the age shown in figures 

 3 and 3a. X 43. 

 Figures 311 and 30. Two sagittal section of an embryo of the same stage as 

 figures 3 and 3a. X 43. 



4 and 4c7. 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. 



5 and 5a. Dorsal and ventral views respectively of an embryo of 



