64 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
is shown for the quadrants A, B, and C in Plate 8, Figs. 69 and 70; 
in d" in Plate 7, Fig. 58; in d'5 in Plate 8, Fig. 66; in d" and d', 
in Fig. 67.) 
Still other divisions in which the spindle lies in the short axis bave 
been followed out in the deseriptive portion of this paper. 
We must conclude, therefore, that a very large number of cell di- 
visions in the cleavage of Asplanchna directly contradict Hertwig’s law 
that the spindle during division comes to lie in such a position that its 
axis coincides with the greatest axis of the protoplasmic mass. The 
characteristic feature of the cleavage is regularity in the direction of the 
spindles, coupled with great variation in the form of the cells, thus 
excluding any close relation between these phenomena. 
What is the evidence upon which this law has been based ? 
It is chiefly experimental, though there is likewise a certain amount 
of evidence based upon the observation of normal cleavage. 
Let us consider first the evidence derived from experiment. The ex- 
perimental studies of Pflüger (84), Roux (85), Driesch (92), Hertwig 
(93*), Born (93 and 794), Ryder (793), and Ziegler (904), on the effects 
of pressure upon the direction of the spindle, are so well known that it is 
not necessary to review them in detail. It is sufficient to state the general 
result. With rare exceptions it has been found that when the egg or 
the cleavage cell is so modified in form that one of the axes which may 
be passed through its protoplasmic mass is distinctly greater than the 
others, the spindle at cleavage comes to lie in this axis. I do not pro- 
pose to enter upon an analysis of these experiments, nor to attempt 
to explain in any different manner the results gained. A study of the 
works of the authors above cited, and a repetition of the pressure ex- 
periments upon the eges of the toad (Bufo lentiginosus Shaw) during 
the spring of 1895, have convinced me that the explanation commorily 
given is the one most in agreement with the conditions, and, from the 
evidence, most probably correct, for these cases. But whatever we may 
hold as to the validity of the explanation for these cases, we know that 
the principle upon which it is based cannot be generalized, since in 
many other cases it is directly contradicted by the facts. Before sng- 
gesting how the experimental results may perhaps be reconciled with 
the apparently contradictory phenomena observed in other cases, it will 
be necessary to consider the evidence gained by other means, as well as 
such experimental evidence as is against the principle. 
First, then, we have the fact that the experimental evidence itself is 
not concordant upon this point. Roux (85) found that under certain 
