78 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



rial for an interesting comparison with that of its near ally, Unio. In the 

 latter case the cleavage is spiral (Lillie '93), and the mesoderm (adult) 

 comes, as in other cases of spiral cleavage, from cZ^-^. In Cyclas, on the 

 other hand, is found a unique type of cleavage, mesenchyma appearing 

 early, possibly at the seventh generation ; but the " mesoderm," as dis- 

 tinguished from the mesenchyma cells, appears much later, and is not 

 separated from the entoderm before its bilateral division. Such a case 

 as this shakes one's faith in homologies based on forms of cleavage or 

 cell lineage. Indeed, it seems impossible in the face of these conflicting 

 results to assign to these phases of embryonic development any definite 

 phyllogenetic significance. On the other hand, the identity of the 

 cleavage processes among certain of the Mollusca (Umbrella, Unio, Cre- 

 pidula, Neritina, and Limax), and the similarity of cleavage in these 

 to that of an entirely difl'erent group of animals, viz. the Annelids, are 

 phenomena not easily banished from the thought. They must have some 

 significance, some common cause. To my mind the appeal to simple me- 

 chanical principles as an explanation of the phenomenon which, broadly 

 speaking, we call the spiral, or alternating, type of cleavage, affords little 

 satisfaction. If the principle of " the resumption of the spherical form," 

 or that of *' minimal surfiices of contact," prevails in one egg, why should 

 it not in all eggs ? We find the spiral type occurring m eggs with no, 

 with little, or with much yolk, and the yolk, when present, variously 

 distributed in the blastomeres ; yet the spirals . occur with absolute 

 certainty and in a definite manner. Other eggs, presenting apparently 

 the same mechanical conditions, cleave in accordance with an entirely 

 difl'erent system, radial or bilateral, in both of which adaptations to 

 mutual pressure may occur without a distinct spiral. 



"VVe can find a satisfactory explanation of the bilateral type of cleav- 

 age. It is simply an accelerated victory of a force which sooner or later 

 dominates every developing egg of the Bilateria. Thus it is tliat the 

 spiral type itself gives w\ay to the bilateral, as Wilson has so well shown 

 in Nereis. 



It must be evident to all that the spiral type is very prevalent among 

 the Trochozoa, i. e. among forms in which a free-swimming larva is early 

 developed. 



Thus, in Nereis at the thirty-eight-cell stage, not only are the germ 

 layers completely differentiated, but most of the individual blastomeres 

 are set apart as protoblasts from which definite organs or parts of the 

 adult body are soon to arise. This occurs about five hours after fertili- 

 zation, and at ten to eleven hours after tliat event the larva begins to 



