376 BERTRAM G. SMITH 



tricity in the superficial cleavage pattern of the early blastula is 

 an expression of the definitive bilateral symmetry of the embryo. 

 As we shall see, a somewhat similar condition exists in the lower 

 hemisphere of the late blastula (figs. 26 and 27), and this is un- 

 doubtedly an expression of the definitive bilateral symmetry; 

 but between these two stages, early and late, respectively, there 

 intervenes a period in which it is more often impossible to detect 

 any deviation from strict radial symmetry in the superficial 

 cleavage pattern. Consequently, we should not assume that 

 there is genetic continuity between these two similar phases that 

 occur at widely separated stages; they require separate investiga- 

 tion. In this section we shall consider only the problematical 

 bilaterality of the early blastula. 



1. Relation to the median plane of the gastrula. a. Orientation 

 experiments. Eighty-three eggs showing excentricity in the 

 cleavage pattern of the early blastula (stages 5 to 7, inclusive) 

 were oriented, each in a separate watch-glass, without removal 

 from their envelopes. Seventy-five eggs Hved to the gastrula 

 stage. The results, which were recorded by means of diagrams, 

 indicate that there is no constant relation between the axis of 

 excentricity in the superficial cleavage pattern of the early 

 blastula and the median plane of the gastrula. This result was 

 wholly unexpected and difficult to reconcile with the impres- 

 sions gained through the study of the internal development; con- 

 sequently, the subject was investigated again by another method. 



h. Staining with Nile-blue sulphate, A method more accurate 

 than the preceding is to remove the envelopes and mark the axis 

 of excentricity by means of a vital stain. The marking was 

 readily accompHshed by applying a small drop of Nile-blue sul- 

 phate to the side of the egg on which the larger micromeres oc- 

 curred. The axis of excentricity is thus defined by an imaginary 

 line drawn from the point marked, through the animal pole. To 

 insure accuracy in the identification of this axis, each egg was 

 examined with a dissecting lens, and those faihng to show marked 

 excentricity were rejected. The results were recorded promptly 

 at the very beginning of gastrulation, to avoid possible errors due 

 to concrescence. 



