388 BERTEAM G. SMITH 



occurs; nevertheless, there are reasons for beheving that this is 

 the side that eventually becomes the thinner (dorsal) side. 

 For in the very late blastula stage (figs. 30 and 31), the side with 

 the thinner wall is in the more advanced stage of differentiation, 

 as evidenced by the columnar character of its cells and the greater 

 progress made in the extension of the cap of micromeres over the 

 yolk cells. In the earher stages mentioned, the depression or 

 crevice in the floor of the blastocoele, if present at all, is located 

 on the side of greater thickness and more advanced differentia- 

 tion. Another evidence of a reversal in the relative thickness 

 of opposite sides of the roof of the blastocoele is that in an in- 

 termediate stage (early phase of stage 10) they are equal in thick- 

 ness, and can then be distinguished only by a careful study of 

 the character of the cells and by the position of the crevice, which 

 in this stage is a fairly pronounced and constant feature. The 

 thinning-out of the originally thicker portion of the roof of the 

 blastocoele is apparently accomplished in part by a migration 

 of cells from its inner surface, away from the median plane, in 

 part by a process of circumcrescence. Since the plates were 

 prepared, I have sectioned additional material in which the gap 

 between figure 25 and figure 30 is more satisfactorily bridged 

 and the above conclusions confirmed. 



1. Relation to the bilateral symmetry of the superficial cleavage 

 pattern. When an egg in a late blastula stage is sectioned along 

 the axis of bilateraUty indicated by the superficial cleavage 

 pattern of the lower hemisphere, this axis is found to coincide in 

 direction with the median plane of symmetry of the internal 

 structure. Thus the bilateral symmetry of the cleavage pattern 

 is but the external expression of the more fundamental symmetry 

 of the internal organization of the egg. 



2. Relation to the bilateral symmetry of the early gastrula. In 

 the living egg, it may be observed that the blastopore begins to 

 form just below the equator, on the side where the lateral wall of 

 the blas.tocoele is more translucent, but at a lower level than 

 the floor of the blastocoele. The study of sections of the be- 

 ginning gastrula shows that the bilaterality of the late blastula 

 is carried over into the gastrula stage (figs. 32 and 33) . Internally, 



