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'-^ in Plate 8, Fig. G6 : in d''' and rf'-«, 

 in Fig. G7.) 



Still other divisions in which the spindle lies in the short axis have 

 been followed out in the descriptive 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 Pfliiger ('84), Roux ('85), Driesch ('92), Hertwig 

 ('93"), Born ('9.3 and '94), Eyder ('93), and Ziegler ('94), 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 a^g 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 eggs of the toad (Bufo lentiginosus Shaw) during 

 the spring of 1895, have convinced me that the explanation commonly 

 given is the one most in agreement with the conditions, and, from the 

 evidence, most probaVjly 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 sug- 

 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 



