CLEAVAGE AND DEVELOPMENTAL PATTERN 565 



ized in the apical regions of the macromeres, and the micromeres are 

 formed from it. Spek holds that this fixed localization results from a 

 marked increase in cytoplasmic viscosity following the third cleavage. His 

 observations lead him to the conclusion that the egg is not a mosaic, 

 though different substances are undoubtedly present in it. It seems evi- 

 dent that this orderly series of events in early cleavages does not indicate 

 independence of parts. The mosaic condition is brought about, according 

 to Spek, dynamically during cleavage through the activities associated 

 with mitosis and the changes in viscosity. To what extent the ectoplasmic 

 changes result from actual migration of the ectoplasm, as distinguished 

 from regional separation of cytoplasm proper from yolk or other inclu- 

 sions, in consequence of altered physical, associated with metabohc, con- 

 ditions, is perhaps still a question. Is the ectoplasm associated with the 

 cleavage furrow, or is that finally localized in the apical regions of the mac- 

 romeres, necessarily the same ectoplasm that originally extended over the 

 egg surface? 



The four groups of apical micromeres spread over the macromeres and 

 give rise to the apical nervous system and sense organ, the swimming- 

 plate rows, two rows from the cells of each group, general ectoderm, and 

 stomodeum. The eight macromeres divide into sixteen entoderm cells, and 

 each gives off a micromere basally which contains the last traces of the 

 green ectoplasm (Spek) but becomes entoderm. Later an entodermal 

 pouch develops in each quadrant and divides into two canals, each leading 

 to a plate row. 



There is no directly visible evidence of polarity in the cytoplasm of the 

 undivided egg, but the relation of the growing oocyte to the parent body 

 (Chun, 1880) suggests the possible determination of polarity by the dif- 

 ferential resulting from this relation. On the other hand, it is possible that 

 position of nucleus near the egg surface is not definitely predetermined 

 but is a matter of chance, and that polar-body formation there constitutes 

 a step in development of polarity, the orderly behavior of ectoplasm in re- 

 lation to this region another step, and its apical localization the final stage 

 in establishment of polar pattern. That the bisymmetrical pattern is also 

 determined dynamically in relation to the ectoplasmic activities associ- 

 ated with first and second cleavages seems not improbable. 



Cleavage and development of isolated parts of the ctenophore egg, 

 blastomeres, and blastomere groups have been studied by a number of in- 

 vestigators.'' In isolated parts of undivided eggs relation of plane of sec- 



•- References given in footnote 11, p. 563. 



