JENNINGS: DEVELOPMENT OF ASPLANCHNA IIERRICKII. 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 direetly proves that in Asplanchna 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 
reine 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 with a different fate of the differ- 
ing cells. This Boveri has shown for Ascaris megalocephala (Boveri, 94), 
and Meyer (795) for certain other nematodes. 
(2) It is directly proved by observation that in certain cases tho cyto- 
plasm of the different cleavage cells in early 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 different 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- 
