282 WATASE. [Vol. IV. 



polar extremity (Fig. XI), and at the next moment the wings, 

 as it were, of the cytoplasm on both sides of the segmentation 

 nucleus become perfectly identical in outline (Fig. X). By 

 rolling the Qgg, we are constantly brought to view the same 

 alternation of facts with perfect regularity. 



Coupled with these phenomena is another presented by the 

 general outline of the ovum. When the wings of the cytoplasm 

 on both sides of the segmentation nucleus appear unequal 

 (Fig. XI), then the whole outline of the ovum corresponding 

 to the above is also unequal, the longer arm of the germ-wing 

 being accompanied by the broader, convex outline (A), while 

 the side corresponding to the shorter wing of the cytoplasm 

 descends rather abruptly (P). By a further study of the phe- 

 nomena through successive stages of development, it can be 

 demonstrated that this more convex border corresponds to the 

 anterior, and the less convex border corresponds to the posterior, 

 side of the egg (Figs. XII, XIII). The view (Fig. X) in which 

 the wings of the cytoplasm are equal is the one in which the 

 optical axis of observation coincides with the antero-posterior 

 axis of the ovum ; hence the two wings correspond to the right 

 and the left sides of the organism (R, L, Fig. X). The view 

 in which the distribution of the cytoplasm around the nucleus 

 is unequal is the one in which the optical axis of observation 

 coincides with the transverse axis of the ovum (Fig. XI). 

 The longer wing of the cytoplasm corresponds to the anterior 

 border of the ovum (A), and the shorter wing corresponds to 

 the posterior border of the same (P). Thus in the unsegmented 

 ovum of the Loltgo, the surface study alone indicates that there 

 is an anterior border (A), different from the posterior (P), and 

 with it there is a right (R) and left side (L) of the organism. As 

 the later embryological studies abundantly show, the end where 

 the germ-protoplasm is situated corresponds to the dorsal side 

 (D), and the opposite pole, where the food-yolk predominates, 

 is the ventral side (V) of the organism. 



This surface observation, when combined with the study of 

 the blastoderm completely separated from the bulk of food-yolk, 

 makes the point still more clear. Such a specimen is shown 

 in Fig. 1 6, PI. X. The germ shows four well-marked concen- 

 tric zones. The segmentation nucleus occupies the innermost 

 circle. Next around it comes a zone of clear substance in 



