MUSEUM OF COMPAKATIVE ZOOLOGY. 259 



to an axis passing through float and distal rim of the elevation, and 

 that opposite the left, for reasons which will soon appear. 



The larva is now a little over two days old. The many cleavage 

 planes, e± pi., forming the polygonal segmentation spheres in the yolli are 

 clearly defined. The protoplasmic network, vt. c, throughout the ovum, 

 is likewise still well marked. The outer of the two layers, or the epi- 

 blast, is ciliated externally. At the elevation on the germinative pole it 

 has a reddish color. The layer beneath the epiblast, or the hypoblast, 

 is thinner than the more superficial. A horizontal diameter of the egg 

 is .45 mm. ; the longest axis at right angles to it, and passing through 

 the germinative pole, is .55 mm. Both epiblast and hypoblast together 

 at the thickest point are not more than .01 mm. in thickness. 



In a slightly older larva (PI. TIL fig. 5) the significance of the notch 

 at the distal rim of the primitive elevation of the germinative area be- 

 comes more apparent. The epiblast and hypoblast, formerly of about 

 the same thickness, have in this stage somewhat changed their relative 

 dimensions, and when seen in profile are observed to have assumed folds 

 which are of significance in the shape of the future covering-scale. The 

 epiblast on the distal side of the disk-like elevation has thickened, and 

 two well-marked angles appear on its exterior. Its sui-face on the distal 

 side rises by a sm.aller angle from the yolk surface than on the proximal, 

 and slopes away more gradually to the opposite side. The hypoblast 

 hugs the yolk cells at all points except at one place {c.x>'l.), where it 

 rises from them, leaving a recess which is later the cavity of the primi- 

 tive larva. Near by this cavity the hypoblast is slightly separated 

 from its enveloping layer, the epiblast, by a middle or third layer. The 

 two angles found on the surface of the epiblast at the distal rim of the 

 forming disk have grown more prominent, as shown in the two following 

 sketches (tigs. 6, 7), and the twa layers have separated more and more 

 from each other. 



When looking at the egg in its present stage of development, we no- 

 tice at once how sharp the difference is between the proximal and distal 

 portions of the rim of the disk-like elevation. They differ very much in 

 shape from each other ; and this difference is magnified as we follow the 

 course of the development into older larvse. In the light of what is 

 known of the existence of bilateral symmetry in the adult Agalnia, we 

 may regai'd this difference in the two borders of the scale as among the 

 earliest expressions of that condition. The forming disk possesses a 

 proximal and distal border, and therefore a right and left side, as referred 

 to a line passing through these regions. This line lies in the same plane 



