DESCRIPTION OF FIGURES i-8, PLATES 1-15 



The surface views were drawn, under the author's direction, by Miss C. M. Reese. The 

 first two of these views were copied, by permission, from S. F. Clarke; the others were 

 drawn from the specimens themselves. 



All of the figures of any one stage are given the same number, followed by distinguish- 

 ing letters, so that it is possible to tell at a glance what figures belong together. 



All of the figures except those from Clarke were drawn under a camera lucida. 



Figure i. A surface view of an embrj'o, from the dorsal aspect, at the 

 beginning of the formation of the enteron. 



Figure ia. A sagittal section of an embryo of approximately the age of the 

 one shown in figure I. X 43- 



Figure 2. A dorsal view of an embryo with hve pairs of mesoblastic 

 somites. 



Figure 2A. A sagittal section of an embryo of the stage shown in figure 2. 



X43- 



Figure 2B. A transverse section through the headfold of an embryo 01 the 



stage shown in figure 2. X 43- 



Figure 3. A dorsal view of an embryo with about fifteen pairs of somites. 



X20. 



Figures 3A-3D. A series of transverse sections through an embryo of the 

 stage of the one shown in figure 3. X 43- 



Figure 4. A surface view of an embryo with about twenty pairs of somites. 

 X (about) 15- 



Figures 4A-4D. A series of transverse sections through the anterior end of 

 an embryo of the approximate age of the one shown in figure 4. X 20. 



Figures 4E and 4F. Two transverse sections through the thyroid gland of 

 this stage; more highly magnified. X 102. 



Figures 4G-4M. A series of transverse sections caudad to the preceding. 

 Figure 4H, X 43 ; other figures, X 20. 



Figure 5. A surface view, in profile, of an embryo at the time of the origin 

 of the limbs. X (about) 5. 



Figures 5A-51. A series of transverse sections through an embryo of the 

 age shown in figure 5. X 7- 



Figure 5J. A composite drawing of reconstructions of the enterons of two 

 embryos of the age of the one shown in figure 5. One reconstruction was in 

 wax, from sagittal sections, the other was a plotted reconstruction from 

 transverse sections. X I4- 



Figure 6. A surface view, in profile, of an embryo with well developed 

 nianus and pes. X (about) 5. 



Figure 6a. A reconstruction, plotted from transverse sections, of the 

 enteron of an embryo of about the age of the one shown in figure 6. X I4- 



Figures 6b-6h. Part of a series of transverse sections from which the 

 preceding reconstruction was made. X 7- 



Figure 7. A reconstruction of the enteron of an embryo of 42 mm. crown- 

 rump length. 



Figures 7A-7G. A part of the series of transverse sections from which the 

 preceding reconstruction was made. X 7- 



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