PLATE 



EXPLANATION OF FIGURES 



49 Median section of a 169 celled stage. X 1134. 



50 Section through a 221 celled stage. Note that some of the cells have 

 become elongated and spindle shaped. Forty hours. X 1134. 



51 Part of a lateral section from the same series, showing certain blastomeres 

 arranged in the form of a nest. X 1167. 



52 Oblique section through a young polygerm. The embryonic cells have 

 already differentiated into two kinds; 1) the definitive or true embryonic cells 

 iP.E.C.) and 2) the inner membrane nuclei (I.M.C). The true embryonic cells, 

 which take a deeper stain, are in the process of forming primary masses. X 1052. 



53 Section through a completed polygerm, showing a single large asexual 

 embryo {As.E.) and several primary masses {Pr.M.). X 612. 



56 



