yS CON KLIN. [Vol. XIII. 



that the micromeres are more firmly bound to each other than 

 to the macromeres, and the explanation of this fact cannot be 

 found in this case in the presence of a segmentation cavity, 

 since this cavity has long before completely disappeared. 

 Although it has not been mentioned before, it will be seen by 

 consulting the figures that before this general rotation of the 

 upper pole takes place (Fig. 33) the ectoblast on the posterior 

 side of the &gg has become bilaterally symmetrical. There is 

 formed at the upper pole, as will be described later, a cross of 

 ectoblast cells, the four arms of which lie nearly half way 

 between the first and second furrows, and hence in the median 

 planes of the macromeres. These arms at first consist of two 

 cells each. Figs. 29, 30, but in the stage represented in Fig. 

 31, in all the arms except one the number of cells is increased 

 to three ; this one, which lies over the left posterior macromere, 

 contains for a very considerable period only two cells. This is 

 one of the first traces of a bilateral arrangement of the micro- 

 meres, though it soon becomes very well marked. In C. adunca 

 bilateral symmetry does not appear in the ectoblast until still 

 later, three cells being formed in the posterior arm of the cross 

 as in each of the others. In the egg of this species, in which the 

 larval history is most completely suppressed, and which might, 

 therefore, be supposed to have adult characters impressed upon 

 it at an earlier period than in eggs with a larval development, 

 bilaterality appears later than in either of the other species. 



Almost from the earliest appearance of the mesentoblast it 

 is in itself bilaterally symmetrical. The divisions which lead 

 to the formation of the primary enteroblasts and the primary 

 mesoblasts are, as we have seen, typically bilateral, and their 

 plane of symmetry very nearly coincides with that of the 

 ectoblast cells. 



Although the macromeres have from the first been radially 

 symmetrical, as is shown by the presence of the polar furrow, 

 yet the future plane of bilateral symmetry is well marked in 

 them, since the first and second cleavage planes which separate 

 them lie respectively in the transverse and median planes of 

 the embryo ; the plane of bilateral symmetry in the entoblast 

 lies at an angle of nearly 45° with that of the ectoblast and 



