BULLETIN: MUSEUM OF COMPARATIVE ZOÜLOGY. 
78 
rial for an interesting comparison with that of its near ally, Unio. In the 
laiter case the cleavage is spiral (Lillie '93), and the mesoderm (adult) 
comes, as in other cases of spiral cleavage, from d”? 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 
phyllogenetio 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 different group of animals, viz. the Annelids, are 
phenomena not easily banished from the thought. "hey 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 cleavago, affords little 
satisfaction. If the principle of * the resumption of the spherical form," 
or that of “minimal surfaces of contact,” prevails in one egg, why should 
it not in all eggs? We find the spiral type occurring in 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 
different system, radial or bilateral, in both of which adaptations to 
mutual pressure may occur without a distinct spiral. 
We 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 that the 
spiral type itself gives way 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-cight-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 that event the larva bogins to 
