80 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
VI.) with Wilson’s Plate XV. Fig. 23 shows at once the great difference 
in the shape of the egg, and the mechanical environment of the cells 
under consideration. The evidence in this case, therefore, seems to 
point to some other force than that of mechanical condition as the 
determining cause of this remarkable agreement. 
The intimate association of the spiral and bilateral types of cleavage, 
and also the prevalence of spiral cleavage in those animals possessing 
precociously developed larval forms, in which bilateral symmetry and 
histological differentiation are early impressed upon the cleaving ovum, 
suggest that the cause of spiral cleavage does not lie entirely in the 
external mechanical environment of the cells, but is, in part at least, 
to be referred to the same “morphogenic force” which produces the 
bilateral symmetry of the embryo and the adult, That the ultimate 
fate of cells exercises a profound influence upon their cleavage is well 
shown in the precocious cleavage of the mesoderm quadrant in Nereis 
and Unio, and of the teloblasts of the larval excretory organs in 
Umbrella. It, may be that in like manner spiral cleavage itself is 
but a manifestation of precocious development of the organism as a 
whole. 
It is also difficult to explain the alternation of spirals by the mechan- 
ical conditions attending their formation. A glance at the tables of 
cleavage which I have given will quickly suggest that, although we have 
the same spiral in a given division of any generation in all the eggs 
having spiral cleavage, the conditions under which the spiral is formed 
in the eggs of different animals are by no means identical. The chrono- 
logical sequence of the division of quartets in different eggs is not the 
same; neither is the distribution of the yolk, either in quantity or 
quality. The spirals however are always identical wherever they occur. 
These external mechanical conditions have doubtless a profound influ- 
ence, but are they the only or the prevailing ones? If we predicate 
this, we must maintain that the resultants of these variously combined 
mechanical influences are identical in all cases of identical spirals, Be 
the cause of the spiral what it may, the internal conditions of nuclear 
division seem to be correlated with the alternation in direction in suc- 
cessive generations. In an unimpeded field of action, the division and 
subsequent equal migration of the two daughter centrosomes would 
necessarily produce a series of cell divisions at right angles to one 
another. This element is doubtless one of the factors in that field of 
complex activities, the cleaving ovum. 
