2l8 CONK LIN, 



EXPLANATION OF PLATE V. 



Fig. 49. Division of the right and left middle cells in the arms a, b, and c, and 

 the expansion of these arms into a cell plate. The posterior arm is much elon- 

 gated, and the posterior turrets and adjoining cells are much enlarged, forming the 

 posterior cell plate. The apical cells lie anterior to the polar furrow. 



Fig. 50. Apical view, showing the cross in heavy outline. The anterior turret 

 cells (la^ and ib^) have divided, and their products, together with those of 2b'-^-^-S 

 form a belt of six cells around the anterior side of the cross ; this belt, together 

 with the tip cells of the right and left arms (aa'-'-', 2a'-'--, and 2bi-'-', 2b'-'-^) form 

 the first velar cell row, or prototroch. The posterior turrets (ic^ and id^) are 

 undivided. (See Note, p. 204.) 



Fig. 51. Bilateral division of the outer rosette cells (la'-'-^, etc.), forming four 

 " rosette series." The last stage in which the polar bodies remain attached. 

 Continued anterior shifting of the apical pole. 



Fig. 52. Ventral view of nearly the same stage as preceding, showing quad- 

 rangular blastopore, the enteroblasts in its posterior angle. 



Fig. 53. Division of the right, left, and posterior apical cells and of the right 

 and left basal cells in the transverse arms ; the basal cells of the anterior arm 

 have already divided. 



Fig. 54. Ventral view of a slightly older stage, showing the narrowing of the 

 blastopore and the division of the macromeres A and C. 



Fig. 55. Dorsal view, showing great enlargement of the proximal cells of the 

 posterior arm, and the continued forward shift of the apex. 



Fig. 56. Apical view of about same stage as preceding figure. Cross shown 

 in heavy outline. First velar row (prototroch) surrounds cross on anterior side. 



Figs. 57-60. The ectoblast has been omitted from these figures in order to 

 show more clearly the entoblast cells. The extent of overgrowth of the ectoblast 

 is indicated by the margin of the blastopore. 



Fig. 57. Formation of the fifth quartette ; 5 A and 5B on the ventral side of A 

 and B, 5C posterior to C. Antero-posterior elongation especially on right side 

 (left in figures). 



Fig. 58. Fourth-quartette cells (4A and 4C) dividing. 4B divides immediately 

 after. 



Fig. 59. Fourth quartette (4A, 4B, 4C) divided ; macromere D dividing. 

 Turning of the posterior end to the left and beginning of final asymmetry. 



Fig. 60. Completion of the sth quartette by the formation of 5D, which lies 

 opposite 5C. Division of the enteroblast E^ 



