CLEAVAGE AND DEVELOPMENTAL PATTERN 587 



development is normal (Spooner, 191 1). Does cleavage pattern determine 

 developmental pattern in these cases, as it seems to normally? 



Ascidian developmental pattern may be much altered by centrifuging, 

 either by displacement of cytoplasmic components and resulting altered 

 localization of particular differentiations or by abnormal cleavage pattern 

 or by both. Neural tissue, sensory pigment cells, notochord cells, muscle 

 cells, and cells resembling entoderm may be found, after centrifuging, in 

 various regions quite different from the normal (Duesberg, 1926; Conklin, 



1931)- 



Regional differences in specific gravity in the egg of the sea urchin 

 Arbacia are not sufficient to determine orientation in the centrifuge. When 

 the egg is centrifuged after fertilization, the first cleavage plane is verti- 

 cal, the second parallel, the third again vertical to the plane of stratifica- 

 tion; and the micromeres, normally at the basal pole, form about the 

 intersection of two of the three cleavage planes, but their position may be 

 centrifugal or centripetal or lateral in relation to the stratification, and the 

 stratified constituents are differently distributed in different larvae, de- 

 velopment being normal. Polarity of egg and embryo is evidently inde- 

 pendent of the stratification.^^ 



With strong centrifugal force and a medium of proper density (sugar 

 solution), unfertilized eggs of Arbacia undergo stratification, elongate in 

 direction of the force, become dumbbell shape, and then separate into a 

 colorless and a pigmented portion. In spherical stratified eggs, the first 

 cleavage plane is usually perpendicular, in elongated eggs, usually parallel 

 to the stratification. The dumbbell-shaped eggs, when fertilized, give nor- 

 mal plutei. Both colorless and pigmented portions can be fertilized and 

 cleave and may develop to plutei. Both portions can be again separated 

 by further centrifuging into two parts; these cleave on fertilization, but 

 development does not proceed far. Activated anucleate halves or quarters 

 can cleave and form blastulae (E. B. Harvey, 1932, 1936, 1939). 



That the primary developmental pattern of the insect egg is localized 

 in the superficial cytoplasm, or in a certain region of it, seems evident 

 from the superficial character of the blastoderm. Stratification of sub- 

 stances by centrifuging in eggs of certain Coleoptera (Hegner, 1909) and 

 muscid Diptera (Pauli, 1927) before blastoderm formation may be fol- 



^*Lyon, 1906, 1907; Morgan and Lyon, 1907; Morgan and Spooner, 1909. Morgan and 

 Spooner believe that the original polarity may be somewhat altered by the effect of stratifica- 

 tion on cleavage but that position of micromeres and larval polarity is as nearly coincident 

 with the original axis as altered cleavage permits. 



