Localization of the Nemertine Egg, 323 



embryo developed from the lower one-third of the same blastula 

 developed no apical organ. 



6. General discussion. 



The points brought out by the present experiments are of con- 

 siderable general interest. In the first place in agreement with the 

 results of Professor Wilson on C. lacteus, it is found that while an 

 egg fragment of an unfertilized egg segments as a whole an iso- 

 lated blastomere of the two-celled stage segments as a half. In the 

 intermediate stages there is a gradually increasing departure from 

 a whole cleavage in the fragments as we pass from the first men- 

 tioned stage to the latter. This is contrary to the statement made 

 by Yatsu in his recently published paper on C. lacteus. In prin- 

 ciple, however, it agrees with the progressive increase of defects 

 found by him in larvae developing from fragments taken at simi- 

 lar stages. 



Though the observations naturally suggest the view that there 

 is a progressive localization of materials in the egg from one 

 period to the other, such a conclusion does not necessarily follow 

 from the experiments themselves without further data. Because, 

 considering the power of regulation of the embryo shown at all 

 stages studied, it must be admitted that there remains the possi- 

 bility of regulation of the unfertilized fragment to form a com- 

 plete whole cleavage and later a complete larva. For the earlier 

 the operation be performed the greater the time which must elapse 

 before the fragment divides, and consequently the greater the 

 chance for regulation to a whole cleavage pattern. The experi- 

 ments of Schultze on inversion of whole frog's eggs at the two- 

 celled stage and the corresponding ones of Morgan on the isolated 

 blastomeres of the same stage, show that the rearrangement of 

 materials due to difference in specific gravity gives opportunities 

 for regulation to a whole development. Observations on the nor- 

 mal eggs of a great many animals during the maturation period 

 show a very extensive series of streaming movements in the proto- 

 plasm at this time. May not these furnish a similar opportunity 

 for regulation to a whole cleavage and whole development? For 



