CLEAVAGE AND DEVELOPMENTAL PATTERN 585 



ticularly clear in eggs of certain annelids and mollusks, in which both a 

 normal cleavage pattern and normal larval differentiation may be quite 

 independent of the axis of stratification.'" Eggs of the gasteropod Ilyanas- 

 sa, centrifuged in reverse orientation, form the temporary polar lobe, al- 

 though, except presumably for the cortex, the cytoplasmic contents of the 

 lobe are quite different from its normal contents. In eggs separated into 

 two parts by centrifuging, the lobe forms in the basal part, and rhythmic 

 changes occur, even though no nucleus is present. Lobe formation and 

 activity are apparently associated with conditions in the cortex, which is 

 not displaced in centrifuging (Morgan, 1935, 1936). With sufficient centri- 

 fuging, eggs of the gasteropod Physa become greatly elongated and sepa- 

 rate into pieces corresponding more or less closely to the stratification. 

 Nucleated pieces, consisting only of clear protoplasm, may develop to 

 young normal snails and hatch (Clement, 1938). 



Effects of centrifuging on developmental pattern are apparently greater 

 in eggs of the oligochete Tubifex and the leech Clepsine. The polar plasms 

 (p. 550) become partly or wholly mixed with other cytoplasmic com- 

 ponents, and development is not normal. '^ These eggs orient in the centri- 

 fuge with apical pole centrifugal; but, according to Schleip, the ectoplasm 

 or cortical layer with the denser parts of the polar plasms attached to it 

 becomes secondarily oriented independently of the entoplasmic stratifi- 

 cation, so that a line joining the two polar plasms forms a right angle with 

 the axis of stratification. 



High-speed centrifuging induces or influences ventrodorsality in the 

 egg of the gephyrean Urechis, a form with spiral cleavage, the centrip- 

 etal region tending to be ventral (Pease, 1938). In ultracentrifuged 

 eggs of Cumingia and Chaetopterus cleavage pattern is related to strati- 

 fication, though with wide variation in Cumingia, and polarity and 

 ventrodorsality are apparently determined in relation to cleavage pat- 

 tern (Pease, 1940, "The influence of centrifugal force, etc.," Jour. Exp. 

 Zool., 84). 



With moderate centrifuging preceding cleavage, the Ascaris egg does 

 not orient, the cleavage pattern is entirely independent of the stratifica-^ 

 tion, and the amount and kind of the stratified substances in particular 

 cells differ in different individuals and are without effect on development. 

 Even after separation of "yolk balls" of considerable size, cleavage and 



^■' See, e.g., F. R. Lillie, 1906, 1909; Conklin, 1910; Morgan, 19106. 



^s Tubifex: Penners, 1922, 19246, 1925; von Parseval, 1922. Clepsine: Schleip, 1914a, b, 

 1929, pp. 128-31. 



