262 LILLIE. [Vol. XVII. 



SO often been led astray, on his own admission, by doubt- 

 ful experiments should realize that everything lies in the 

 interpretation. 



One would suppose that even Driesch would admit that the 

 problem of the development of an entire ^gg is no more com- 

 plicated than that of a part ; and that the fact that a part may 

 develop into a normal embryo gives no explanation of how a 

 normal embryo is formed, either in " absolut-normal " or in 

 modified cases. 



The evidence on which I base my argument for a bilateral, 

 and more than bilateral, organization of the cytoplasm of the 

 Qg% of Uiiio may be summarized thus : i. The polar differentia- 

 tion is clear and definite, but without reference to the distribu- 

 tion of yolk. 2. Evidence of a bilateral orientation of the 

 protoplasm is given by the behavior of the germ-nuclei and of 

 the sphere substance ; the line uniting the two germ-nuclei just 

 before they come together is at right angles to the long axis of 

 the future first cleavage spindle, and the plane of elongation of 

 the sphere substance is parallel to the future axis of this 

 spindle. We have, then, two independent phenomena that 

 have definite reference to the future axes of the embryo, for 

 the first cleavage spindle has such a definite relation. In the 

 absence of definite elongation of the sphere substance it would 

 be possible to interpret the determination of the axes of the 

 embryo as due to the plane of meeting or of copulation of the 

 germ-nuclei. The coincidence of two such independent phe- 

 nomena can only be interpreted as due to a third factor, "uiz.: 

 bilateral orientation of the egg-cytoplasm. 3. Evidence of dif- 

 ferentiation of anterior-posterior proportions is found in the 

 shifting of the first cleavage spindle. We have seen that this 

 shifting can be due neither to unequal distribution of the yolk 

 nor to unequal power of the cleavage centers. Moreover, the 

 final position of the spindle has a differential value in the 

 subsequent ontogeny, i.e., it is adapted to the formation of a 

 large shell gland and mesodermal structures, as I showed in 

 my paper on "Adaptation in Cleavage." 



In this paper I pointed out that the only possible basis for 

 a rational interpretation of the rate, direction, and place of 



