JEXXIXGS: DEVELOPMENT OF ASPLAXCHXA HERRICKII. 79 



was a qualitative division, nuclear or cytoplasmic, at the preceding 

 cleavage. 



This conclusion is, of course, speculative, but the history of the 

 cloud of granules directly proves that in Asplanchua the cleavage is 

 accompanied by differentiation. 



The recent experimental evidence, showing that in certain organisms 

 the cleaving cells in early stages all possess the inherent capacity to 

 produce an entire animal, has led to a rather widespread impression 

 that cleavage has been shown to be a process of little or no significance, 

 being merely a quantitative division of a mass into smaller masses of a 

 similar nature. This view apparently receives confirmation from gen- 

 eralized statements of the results of such experiments ; for example, 

 the following from Driesch ('94, p. 69): " Es liegt also nach allem 

 gesagten in der That kein Grund vor, in der Furchung etwas anderes als 

 reiue Zellteilung zu sehen ; ja die Gleichheit der Furchungskerne ist 

 direkt durch Versuche bewiesen." A summary of the evidence which 

 has been adduced in regard to this matter shows that such a statement 

 as the above conveys only a small part of the truth and must lead to 

 error unless carefully interpreted. The evidence that cleavage is accom- 

 panied by differentiation may be summarized as follows. 



(1) It is directly proved by observation that in certain cases the nu- 

 cleus differs in structure in different blastomeres in early cleavage stages, 

 and that this differentiation is correlated loith a different fate of the differ- 

 ing cells. This Boveri has shown for Ascaris megalocephala (Boveri, '94), 

 and Meyer ('9-5) for certain other nematodes. 



(2) It is directly proved by observation that in certain cases the cyto- 

 plasm of the different cleavage cells in eai-ly stages is of a different 

 structure, reacting differently to chemical reagents, and this difference 

 is correlated with a different fate of the different cells. Thus, in the 

 sixteen-cell stage of Nereis, " the somatoblast can always be recognized 

 at a glance " from its different color (Wilson, '92, p. 390). A similar fact 

 has been shown above in regard to the cytoplasmic differentiation in 

 Asplanchna, but here I have not been able to determine the fate of the 

 cell which receives the differentiated granules. 



(.3) It has been shown that in many cases the diffei'ent blastomeres of 

 early cleavage stages give rise to definite structures in the adult. This 

 fact in itself of course admits of two interpretations, but taken in con- 

 nection with the facts stated under (1) and (2) it becomes of great 

 significance. 



(4) It has been shown experimentally, that in some organisms sepa- 



