No. I.] THE EMBRYOLOGY OF CREPIDULA. 167 



direction and occupy the same relative positions : e.g., the 

 cross is formed over the middle of each macromere, but since 

 in all cases it occupies the same position relative to the future 

 axes, while the macromeres may occupy different positions, it 

 follows that it must rotate in some cases and not in others.^ 



Coenogenetically the macromeres have been much modified, 

 witness their differences in size; and the difference in position 

 is probably another such modification. Whether in later stages 

 even these differences disappear is not certainly known, but even 

 if four cells do occupy different positions in different species, it 

 is a small matter compared with the fact that hundreds of cells 

 forming the most important organs of the body occupy the 

 same positions. These facts show that both phylogenetically 

 and ontogenetically the position of the ectomeres and meso- 

 meres is more fundamental than the position of the entomeres. 

 A proper comparison, therefore, between the axes of different 

 animals in these early stages is to be found in the position of 

 the ectoblast and mesoblast rather than in that of the entoblast. 

 Such a comparison would show, I think, that in all annelids 

 and mollusks (excepting, of course, the cephalopods) the median 

 and transverse axes lie in the same groups of ectomeres and 

 mesomeres, and it might possibly unify the varying results 

 thus far obtained in the study of the cleavage in fishes and 

 amphibians. 



2. Establishment of the Larval Axes. 



Hatschek ('88) has expressed in the terms Protaxonia and 

 Heteraxonia the fact that in one great group of animals, the 

 Coelenterata, the primary axis of the ovum becomes the chief 

 axis of the larva and of the adult, while in all animals with 

 bilateral symmetry these two axes are not the same. In the 

 typical trochophore larva, e.g., that of Polygordius, this change 

 of axis is accomplished by the forward movement of the blasto- 



1 In accordance with the description of these rotations given in the earlier part 

 of this paper, I here regard the macromeres as fixed and the micromeres as rotat- 

 ing. In view of the conclusion here reached it would be much better to reverse 

 the process and consider the micromeres fixed in position and the macromeres 

 changeable. 



