76 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
statement is of course utterly without significance. The real question 
is, What determines the point where it divides? Why in Asplanchna 
does the middle of the sphere of action in the fourth and seventh 
cleavages of the large ventral entoderm cell lie in such a position (Fig. 
16, Plate 2, and Fig. 65, Plate 8) as to divide the cell into parts which 
are only slightly unequal, while in the intervening fifth and sixth 
cleavages (Fig. 35, Plate 5, and Fig. 49, Plate G) the middle of the 
sphere of action is at the periphery of tho cell! Why at tho first 
cleavage of the frog's egg is the middle of the sphere of action in the 
centre of the mass of formative protoplasm, whereas at the divisions 
immediately preceding — the formation of the polar cells — it is at the 
periphery of the egg ! 
In regard to the question what determines the comparative rate of 
‘cleavage the case is similar. In view of the recent discussion of these 
two questions — the inequality of cleavage and the rate of cleavage — in 
the works of Kofoid (94, p. 196), McMurrich (795), Lillie (95, p. 45), 
zur Strassen (95), Ziegler (95), and others, it seems scarcely worth 
while to insist upon the fact that the rate of cleavage and the equality 
or inequality of the produets are related to the future morphogenetio 
processes, and show in many cases no relation whatever to accumulations 
of yolk. Yet the so called laws, according to which these matters are 
determined by the distribution of yolk, are repeated in O. Hertwig’s 
text-book on the cell (Hertwig, 93, pp. 174 and 180), and reaffirmed in 
the latest edition of his treatise on embryology (Hertwig, ’96, pp. 67 
and 84). 
In Asplanchna the main facts in regard to the sequence of cleavage 
are as follows. 
(1) The order of cleavage is, within very narrow limits, constant. If 
a number of eggs are taken showing successive stages of the division of 
any given cell, the series will show corresponding successive stages in 
the nuclear history of the other cells. i 
(2) There is a typical order for similar cells of different quadrants of 
the same egg. This order is D, C, B, A. 
(3) There is a marked general tendenoy for larger cells to divide 
first. In every case where two cells of the same origin are of different 
size, the larger divides before the smaller. 
This is a very general phenomenon, as has been repeatedly pointed 
out of late, even in cases where the larger cell contains the greater 
amount of yolk. The “law” recently formulated by Hertwig (96, 
p. 67), that “die Schnelligkeit der Furchung proportional ist der Con- 
