S84 PATTERNS AND PROBLEMS OF DEVELOPMENT 



cases developmental pattern is independent of the stratified distribution 

 of these substances by centrifugal force. Stratification into three, four, or 

 five more or less distinct layers or zones of different specific gravity and 

 often of very different appearance results from centrifuging, according to 

 the material and intensity of force. On cessation of centrifuging a gradual 

 reconstitution of normal cytoplasmic structure usually tends to occur, 

 except after very high intensities. In consequence of displacement of nu- 

 cleus or spindle, one or both polar bodies may form elsewhere than nor- 

 mally, and cleavage pattern may be altered. 



In the Crepidula egg, centrifuged at first polar spindle stages, the spindle 

 may be displaced as a whole or greatly elongated ; and when division oc- 

 curs, the cell which would normally be the polar body may be as large as, 

 or even larger than, the other cell. One or both polar bodies may be en- 

 larged in this way, but even with abnormal size and position of polar 

 bodies cleavage gives evidence of persistence of pattern, presumably the 

 original pattern (Conklin, 1917). 



In some eggs containing much yolk, oriented with polar axis in the 

 direction of the force, apicobasal differences in rate of cleavage may be 

 increased or reversed with aggregation of yolk at one pole or the other. 

 Amphibian eggs centrifuged with heavier vegetal pole centrifugal may 

 approach or attain meroblastic cleavage because the dense aggregation 

 of yolk prevents cleavage in the centrifugal region. When centrifuged in 

 reverse orientation, cleavage may become more rapid in the basal than 

 in the apical region.^-' Such effects are primarily mechanical, and the ag- 

 gregation of yolk may make normal development mechanically impossible ; 

 but this gives no information concerning physiological pattern. However, 

 even with marked stratification in the amphibian egg, the blastopore is 

 normally localized, and development may be essentially normal (Morgan, 

 1906c). Apparently the region of the dorsal lip has not been displaced. 



Many eggs do not become axially oriented to centrifugal force, or orien- 

 tation can be prevented; consequently, stratification may be at any angle 

 to the axiate pattern, but a normal developmental pattern may appear 

 even with a distribution of cytoplasmic components entirely different 

 from the normal and different in different individuals. 



Developmental pattern in the few coelenterate eggs centrifuged is ap- 

 parently quite independent of degrees of stratification or alterations of 

 cleavage resulting (Conklin, 1908; Beckwith, 1914). Complete or almost 

 complete independence of developmental pattern and stratification is par- 



2^0. Hertwig, 18986, 1904; Wetzel, 1904. 



