286 A. RICHARDS. 



EXPLANATION OF PLATE II. 



FIG. n. Same material and stage as preceding. Similar conditions shown. 

 Rest of nucleus in next section. 



FIG. 12. Same. It is to be noted that the division of the sphere has already 

 taken place for the next mitosis. 



FIG. 13. F9 X Cc?, normal material; condition of nucleus before the first 

 cleavage division showing the distinct vesicles. Dotted line marks the area for- 

 merly occupied by this nucleus, shrinkage in preparation for ensuing mitosis having 

 already occurred. 



FIG. 14. F9 X Ccf; radiated for three minutes after fertilization. A nucleus 

 from one-cell stage. 



FIG. 15. F9 XCd"; both radiated for three minutes before fertilization. 

 Typical resting nucleus of second cleavage mitosis. 



FIG. 16. F 9 X Cd"; drawn with 6 objective and 4 ocular to show change in 

 cell polarity. Blastula stage; upper cell is ectodermal; chromosomal vesicles 

 shown in outline. Original axis was in line AB. With the shifting of the position 

 of the spheres in telophase the axis of the upper cell came to line in line CD and 

 that of lower in line EF. 



FIG. 17. F9 XCcf; eggs radiated for fifteen minutes before fertilization. 

 Beginning of prophase of second division. Vesicles all distinct, showing a very 

 definite polarity. Chromatin granules uniformly small and well distributed. This 

 cannot be interpreted as merely a tangential section showing lobations, for the 

 next section shows quite the same conditions. 



FIG. 18. F9 X Fc?; from slide loaned by Miss Pinney as are Figs. 19, 20, 

 21 and 23. First cleavage similar to Fig. 17. 



