100 



BIOLOGICAL LECTURES. 



A comparison of the embryo of Podarke with that of Amphi- 

 trite at the sixty-four-cell stage will be instructive. Fig. 6 

 represents the completion of the sixty-four-cell stage in Podarke 

 by the division of the descendants of the second group of micro- 

 meres. Note especially the small size of the macromeres, 

 4-A, 4-B, 4-C, 4D, the equality in size between the members of 

 the fourth quartette of micromeres, and the arrangement of 

 spindles in the cells undergoing karyokinesis. At this time 

 the embryo of Podarke is apparently radially symmetrical, a 

 symmetry which is only apparent, as is indicated by the position 

 of the "polar furrow." This apparent symmetry is destroyed 



Fig. 5. Amphitrite, sixty-four cells from 

 below ; cp-dp, primary trochoblasts. 



Fig. 6. Podarke, fifty-six to sixty-four 

 cells from below. 



by the completion of the division shown in the figure, separating 

 the small cell X\, 2 , which comes to lie just over^, themesoderm. 



From this time on, bilateral divisions appear among the cells 

 of both hemispheres, the most important being those which 

 lead to the formation of the bilaterally symmetrical cross at the 

 upper pole, and the bilateral division of ^d with the cells of the 

 third quartette of micromeres, which immediately adjoin it at 

 the lower pole. The last division is of especial interest, as we 

 shall see, as it leads to the formation of a larval mesoblast. 



Let us now compare this with Amphitrite. Fig. 5 represents 

 an embryo of Amphitrite at approximately the sixty-four-cell 

 stage. Note the large macromeres, and the large size of 

 2d=X and 4d=M when compared with other members of 



