The Development of the AlUgator 339 



The magnification of each figure,, except those from Clarke, is indi- 

 cated 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. 



Figure i. Surface view of egg. X f. 



la. Egg with part of the shell removed to show the chalky 

 band in the shell membrane. X §• 

 Figures 2 and 2a. Dorsal and ventral views respectively of the 

 blastoderm before 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. 

 36-3m. Transverse sections of an embryo of the age shown 

 in Figures 3 and 3a. X 43- 

 Figures 3M and 30. Two sagittal sections of an embryo of the same 

 stage as Figures 3 and 3a. X 43. 



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. 



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



of almost the same age as the preceding, to show the fur- 

 ther development of the medullary folds. After Clarke. 



Figure 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. X 43. 



Figures "ja-yh. 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 five pairs of mesoblastic somites. X 20. (Drawn 

 by transmitted light.) 



