ii8 



BIOLOGICAL LECTURES. 



The spindles in the trochoblast cells indicate the direction of 

 their next division, which results in the 64-cell stage (Fig. 9). 

 Fig. 10 is an apical view of the anterior hemisphere in the 



32-cell stage, showing the 

 four alternating quartettes 

 of umbrella cells, and the 

 uppermost quartette of sub- 

 umbrella cells. The cells 

 of the first quartette have 

 the nuclei figured ; those of 

 the second are left clear ; 

 those of the third and fourth 

 quartettes (primary trocho- 

 blast cells) are stippled. All 

 of the thirty-two cells divide 

 again, so that the egg is 

 composed of sixty-four cells, 

 arranged in sixteen quar- 

 tettes, making sixteen cells 

 in each quadrant. Four of 

 these quartettes, the fifth, sixth, seventh, and eighth, stippled 

 in Fig. II, comprise the cells of \.\\q primary prototroch. 

 It is obvious that, in consequence of the regularity of cleav- 



FiG. 9. — A mphitrite, 32-cell stage, from anterior end. 

 View of umbrella or anterior hemisphere. 





Dorsal 



0* 



Fig. 10. — A mphiirite, from left side. Primary 

 and secondary, shaded, respectively, with 

 dots and lines. 



Ventral 



Fig. II. — Amphitrite, beyond 32-cell stage, 

 from anterior poles, slightly distorted to 

 show all 25 prototrochal cells. 



