394 F. L. LANDACRE 



EXPLANATION OF FIGURES 



All sections are cut 7 micra in thickness, and all figures and reconstructions are 

 taken from transverse sections. Figures were drawn with a camera lucida with 

 objective 4 mm. eye-piece, X 8 Spencer, reduction to J. Total magnification, 186 

 dia. 



1 (A. neb., Stage I) is taken at a stage when the medullary plate is slightly dif- 

 ferentiated into the neural cord (N. C.) and the lateral mass (L. M.). The sec- 

 tion lies just posterior to the point of formation of the optic cup. 



2 and 3 (A. neb.) are taken from the same embryo, Stage II. Fig. 2 is taken six 

 sections posterior to the optic vesicle and lies in a region just anterior to the point 

 at which the Gasserian ganglion will form later. The lateral mass here breaks 

 down completely into mesectoderm. 



3 is taken just posterior to the future position of the Gasserian ganglion, twenty- 

 nine sections posterior to the optic vesicle. The greater portion of the lateral 

 mass here breaks down into mesectoderm also. 



4 to 9 (A. neb., Stage III) are all from the same embryo which is slightly older 

 than that from which figs. 2 and 3 were taken. Fig. 4 lies four sections anterior 

 to the auditory vesicle and shows the differentiation of the lateral mass into the 

 preauditory placode and the dorsal portion of the lateral mass which later forms 

 the lateralis VII ganglion. 



5 shows the differentiation of the lateral mass into dorso-lateral mass and post- 

 auditory placode, four sections posterior to the auditory vesicle. 



6 is taken through the middle of the auditory vesicle before the appearance of 

 a definite cavity. 



7 lies four sections posterior to the optic vesicle. It is taken through the same 

 relative position as fig. 2, Stage I, and shows the transition of lateral mass into 

 mesectoderm. 



8 is taken through the regioninwhichthe Gasserian ganglion forms. The lateral 

 mass does not break down into a loose mass of cells so early here as it does just 

 anterior and posterior to the Gasserian ganglion. 



9 lies five sections anterior to fig. 4 and nine sections anterior to auditory ves- 

 icle. It shows the reduction in size of the preauditory placode as one reads for- 

 ward from the position of fig. 4. 



