96 



LILLIE. 



EXPLANATION OF PLATE IV. 



Fig. 40. Thirty-one-cell stage. Lower pole. 



Fig. 41. Lower pole. Division of ^ 21. 



Fig. 42. Lower pole. Division of b^.i , b 22 , c 22 and d^. Thirty-seven-cell 

 stage. 



Fig. 43. The same stage from in front. 



Fig. 44. Equal division of X. First bilateral cleavage. Notice relations of 

 jr I , x2, xi, and j; 4 to X. View from behind. 



Fig. 45. Lower pole at the time of the bilateral division of M. 



Fig. 46. View from behind. Formation oi xs. 



Fig. 47. Same stage from the right side. 



Fig. 48. Upper pole of stage of Fig. 44. Division of a ' and a 2.1 . 



Fig. 49. Upper pole. Division of c^ and a'. 



Fig. 50. Upper pole. Sixteen cells of the first generation of ectomeres. 



Fig. 51. Slightly later stage. Division of d 12 . Stage of Fig. 45. Fifty cells. 



