PLATE 4 



EXPLANATION OF FIGURES 



48 Nineteen-cell stage, dorsal view, showing the exact position of all the 

 cleavage cells. 



49 Thirty-three-cell stage, dorsal view; the end of fifth cleavage; d^ is present 

 and has divided; the large cell D (E) has begun to invaginate. 



50 Thirty-five-cell stage, dorsal view; the minute cells d^ and d^ are the 

 additional cells formed. 



51 Thirty-four-cell stage, ventral view. 



52 Thirty-four-cell stage, posterior end of the embryo, to show the position 

 of the closing blastopore. 



53 Thirty-three-cell stage; same view as preceding figure. 



54 Later stage in the closure of the blastopore from the posterior end. 



55 to 57 About fifty-cell stage, showdng the invagination of the large cell E 

 the central cavity into which E passes is indicated by fine stippling. 



58 Sixty-four-cell stage, dorsal view, showing the first cleavage spindle of E. 



59 Same as the preceding from ventral view. 



60 Later stage in which E has divided. 



430 



