2l8 



THE MOSAIC STAGE OF DIFFERENTIATION 



to the centrifuge), the fat forms a layer, and beneath this, in suc- 

 cession, there are layers of clear cytoplasm, yolk, and pink granules, 

 the latter occupying the centrifugal pole^ (fig. 104). 



But in spite of this complete restratification of the visible egg- 

 contents along a new axis, the original axis of polarity has not been 

 affected. The position of the original axis is indicated by the funnel 

 in the jelly which marks the position of the original animal pole of 

 the egg : the micromeres are formed and invagination begins at the 

 opposite (original vegetative) pole, regardless of the visible contents 



Fig. 104 

 Persistence of the primary axis in sea-urchin (Arbacta) eggs in spite of the re- 

 arrangement of visible substances in the cytoplasm. After centrifuging, the egg 

 becomes stratified with fat at the centripetal pole, then clear cytoplasm, then 

 yolk with increasing amounts of pigment. The first cleavage (left top) is always 

 at right angles to the stratification, but the micromeres are always formed at the 

 vegetative end of the original axis, whether this coincides with the centripetal 

 pole of the centrifuged egg (top right), its centrifugal pole (bottom left) or its 

 side (bottom right). (Redrawn after Morgan, Experimental Embryology, Columbia 

 University Press, 1927.) 



which happen to be situated there.'^ Development continues along 

 the lines of the original axis and is normal, from which it follows 

 that the various substances which have been disarranged are not 

 organ-forming. (See also p. 69 as regards determination of 

 bilaterality by centrifuging.) 



Similar results have been obtained from centrifuge experiments 

 on eggs of other animals. The egg of the Lamellibranch Mollusc 

 Cumingia after centrifuging shows a stratification into four zones : 



Lyon, 1906; Morgan and Lyon, 1907. 



Morgan and Spooner, 1909. 



